In a future where the polar ice-caps have melted and Earth is almost entirely submerged, a mutated mariner fights starvation and outlaw "smokers," and reluctantly helps a woman and a young girl try to find dry land.
The polar ice caps have melted, and the earth is covered by water. The remaining people travel the seas, in search of survival. Several different societies exist. The Mariner falls from his customary and solitary existence into having to care for a woman and a young girl while being pursued by the evil forces of the Deacon.
In the future, the Earth is engulfed with water when the polar ice caps melted, leaving the lands and civilization beneath the sea. In "Waterworld" drifters sail across the waters of the world on their man-made ships, trading for supplies and salvaging what is left of civilization, communities of survivors live in towns called "Atolls", and a force of evil pirates "The Smokers" travel across Waterworld in search of a mythical island called "Dryland". Arriving at an Atoll, a mysterious drifter without a name called The Mariner is sentenced to death when the community discovers he is a mutant. When the Atoll is attacked by The Smokers led by their evil leader Deacon, a woman named Helen and her adopted daughter Enola bribe The Mariner to take them to Dryland, believing The Mariner has been there. But the Mariner, Helen and Enola are pursued by Deacon and the Smokers, when The Mariner learns the tattoo on Enola's back is in reality a map that will lead them to Dryland...
Sometime in the future, the polar ice caps have melted and the world's oceans and seas have covered the land. People are few and far between, living in small communities at sea or sailing from one to another as traders. All the people seek dry land.. something nobody has seen.
The world is flooded. Civilisation is lost under the sea. The Mariner sails his trimoran over the seas, drinking his own Urine and visits a floating atoll of "Drifters". When they find the Mariner to be a mutant they sentence him to death. Meanwhile, a girl with supposedly a map to get to dry land tattoo-ed on her back, is the objective for an attack by a gang of smokers who attack the atoll.
- At an unspecified point in the future, a narrator explains, the polar ice caps have melted covering the Earth in an endless ocean. The survivors of the ancient deluge have had to adapt to the new world.
One such survivor, known only as The Mariner (Kevin Costner), travels the globe on his trimaran. He lives by recycling his own urine as drinking water and scavenges for artifacts on the ocean floor using a diving bell with a timed pulley system. While diving, someone boards the Mariner's boat and steals the fruit from his tiny lime tree. The Mariner surfaces in time to see the other boat sailing off and immediately runs to his harpoon, holding the drifter (Chaim Jeraffi) at bay and saying that when two sailors meet it's customary to trade. He adds that he's seen the man's boat but doesn't recognize him. The drifter says he took the boat fair when he found the previous owner dead and tells the Mariner of an atoll a few days away where he can resupply.
Just then, the men spy two jet skis nearby piloted by Smokers, pirates who are so named for their habit of smoking cigarettes and their dependence on oil for fuel. They advance on the goods the Mariner has raised from the depths with the diving bell. Meanwhile, as the drifter sets sail, he reveals he's stolen the Mariner's limes. Infuriated, the Mariner quickly loosens ropes and throws custom switches on his boat, raising the sail and giving him enough speed to collect his bag of goods and catch up to the drifter. He sails right over the man's small boat, breaking the sail, disabling his craft, and leaves him for the Smokers.
After several days the Mariner arrives at the atoll the drifter spoke of. At first he's refused admittance, but the atoll's guards allow him in when he shows them he has a jar full of dirt; the world's most precious substance. Inside the gates, he finds that the people live in poverty and are near starvation. A small group is conducting a funeral where the deceased is 'recycled' and dropped into a pool of viscous waste. The Mariner docks his boat and is met by the atoll's Enforcer (R.D. Call) who gives him a two hour limit to conduct business. The atoll's banker (Jack Kehler) offers him a large amount of currency known as 'chits' for the quality dirt he was carrying, but the Mariner demands twice that amount and is given it.
At the atoll's bar/store, a shady man named Nord (Gerard Murphy) is asking a man (Robert A. Silverman) desperate for a drink of fresh water (hydro) about a girl with a tattoo. Supposedly, the tattoo has markings that give the location of dry land. Nord then spots the girl, Enola (Tina Majorino), who is in the care of the bar's owner, Helen (Jeanne Tripplehorn). Helen quickly hides Enola, but not before Nord catches a glance of the tattoo on the child's back.
The Mariner finds the shop and spends his money on hydro, a tomato plant, and all the shelves in the bar. Nord approaches the Mariner, asking him about his newfound wealth, but the Mariner remains unfriendly. He marches back to his boat and begins to load his purchases on it. A small group of people, led by two elders (Zakes Mokae and Sab Shimono), approach asking if he'll take a young, pregnant woman to sea with him since he has all the supplies he could need. When the Mariner refuses, the elders surmise he must be hiding something and theorize he's a Smoker spy. They quickly detain him and, in the scuffle, one of them discovers gills behind the Mariner's ears. Horrified by the mutation, the crowd attacks the Mariner but he manages to kill one of them in the lagoon before he's caught in a net. Before he can be beaten further, the Enforcer steps in and defends the Mariner, claiming he was acting in self-defense. However, since he will need to pay for his crimes, he will be imprisoned until further notice.
Later that night, while in a cage hanging over the docks, the Mariner spots Nord slipping out of the atoll while a group of youths steal various items from his trimaran. Meanwhile, Helen is with her friend Gregor (Michael Jeter), an eccentric inventor, and says that people are talking too much about Enola and that they should leave for the girl's safety. Gregor is frustrated over his own inability to read the map to dry land on Enola's back. Enola is blissfully drawing pictures, some of which are of things that even she doesn't recognize. She innocently mentions that the Mariner may know where dry land is. Gregor goes out to the Mariner and excitedly inspects his webbed feet and gills. He asks if the Mariner knows where dry land is. The Mariner asks him to pick the lock and release him but their conversation is cut short by the Enforcer and Gregor leaves.
The following morning the council congregates around the Mariner and decides he is to be recycled in the customary fashion. At that moment, a lookout spots an armada of Smokers descending on the atoll. The group, led by the Deacon (Dennis Hopper), launches a full scale attack. In the chaos, Gregor accidentally activates his hot air balloon and is unable to slow the mechanism to allow Helen and Enola aboard. Gregor escapes but Helen and Enola spot the Mariner, whose cage was knocked into the sludgy pit during the battle. Before he drowns, Helen and Enola free him on the condition that he'll take them with him. Working together, they manage to free the Mariner's trimaran and open the gates to the atoll. The Deacon spots them and orders his gunner (Neil Giuntoli) to stop the trimaran, but the Mariner launches his harpoon and drags the gunner's boat so that it fires on the Deacon's pontoon, causing the fuel it's loaded with to explode. The Mariner, Enola, and Helen escape.
Within the wrecked atoll, Nord, who had been the Smoker spy, leads the Deacon to a couple of survivors to interrogate. Having survived the explosion but losing an eye in the process, the Deacon learns that his quarry, the girl with the tattoo, escaped with the Mariner who is a mutant. He alerts his crew of their new mission to find her.
Aboard the trimaran, Helen asks the Mariner if he knows the way to dry land. He hints that he does, but he tells Helen that they'll only make it if they throw Enola overboard. As a child, she's useless and will eat all their food. Helen is adamant about keeping Enola and goes so far to offer herself to the Mariner. However, he refuses her. Angry, Helen grabs a harpoon gun and demands they all go to dry land together, but the Mariner drops his sail on her, pinning her beneath, and knocks her out with an oar. He spends the rest of the day keeping watch from the top of his sail.
Aboard a rusted oil tanker, the Smoker's home base, the Deacon fashions himself an eye-patch when an attempt by the ship's doctor (John Fleck) to give him a false eye fails. He inspects the ship's oil supply maintained by a meter reader (William Preston), passing out cigarettes to his citizens on the way, and discovers that his ship is rapidly losing oil. The Deacon maintains that their main focus is to find the tattooed girl and sends out a plane to search for her.
Soon enough, the pilot and his gunner (Jack Black and John Toles-Bey) spot the trimaran. In a measure to keep their location secret, the Mariner prepares a weapon to fire at the plane below deck. However, Helen isn't clued in to his strategy and impatiently fires the harpoon at the plane, killing the gunner and tethering the plane to the trimaran. The Mariner is forced to climb his sail to cut the rope but the pilot manages to shoot the plane free. The whiplash sends the Mariner plunging into the water. Infuriated, he pins Helen down and viciously cuts her hair, telling her not to touch his stuff. When Enola protests, he cuts her hair too and confiscates the crayons she's taken from him to draw all over the boat.
Having returned to the tanker, the pilot tells the Deacon the Mariner was last seen heading. The Deacon predicts that, since the Mariner knows he's been found, he would expect the Smokers to think he's changed course. However, knowing the Mariner is likely no fool, the Deacon says that his course will remain unchanged, and that's where they will spring their trap.
Some time later the Mariner spots another sailor who has raised a neutral flag. Helen convinces the Mariner to make contact, saying the man might have some food. The drifter (Kim Coates) is clearly neurotic, having spent too long at sea alone, and acts unpredictably. While he and the Mariner trade, Helen tries to fashion a fishing pole. The drifter offers the Mariner something 'saved for a special trade' - paper. The Mariner doesn't have much to offer in return but the drifter asks for half an hour with Helen. Despite her protests, the Mariner agrees but refuses the drifter time with Enola. The drifter and Helen go below deck while the Mariner inspects the paper but, after a few moments, goes down and tells the drifter the deal is off. The drifter reacts violently and Helen goes back up top while the two men fight, with the Mariner coming out victorious. He dumps the drifter's body into the sea and scavenges his boat for useful items. Irritated it came to that, he takes Helen's fishing pole and throws it away, telling her she'll never catch anything with it.
The Mariner takes his harpoon gun and attaches a pulley to it, using it to drag himself behind the boat. Soon, a large sea creature comes up from below and tries to swallow the Mariner, but he uses the harpoon gun to blast holes in the animal. That evening, the three feast on the creature. Enola happily hums to herself and the Mariner suggests she stop making noise long enough to listen to the world. Looking at his feet, Enola mentions how she'd like to have feet like that so she could swim; something she never learned to do. The following day, the Mariner teaches Enola how to swim, an act that impresses Helen and leads to a bond between the Mariner and Enola.
Days later, they come to a slaver outpost. Desperate for repairs, the Mariner tries to communicate with them but the inhabitants don't respond. Using an underwater periscope, the Mariner discovers the slavers have been killed and Smokers are positioned underwater to ambush them. They just barely manage to escape, even after the Deacon fires a shot with his rifle, wounding the Mariner.
Later, Helen asks again about dry land and wants to know when they may be able to find it. The Mariner becomes impatient and says that he's sailed farther than anyone but has never seen dry land. Upset, Helen asks about all the things on his boat that no one has ever seen and wonders where the Mariner got it all. The Mariner agrees to show Helen where 'dry land' is and sets up his diving bell, leaving Enola on the surface. The Mariner swims alongside Helen and takes her to the bottom of the sea where the remains of a city lie submerged. When they surface, Helen admits she never knew the truth, though she'd always heard legends.
Suddenly, the Deacon and a crew of Smokers appear, having already boarded the trimaran. They hold the Mariner and Helen captive and demand to know where the girl is. Enola has hidden herself within the trimaran but when the Deacon bluffs the other's deaths she reveals herself. Enola is taken captive and the trimaran is burned. The Mariner and Helen escape death by submerging themselves and the Mariner kisses Helen so that he can breathe for both of them. When they surface, Enola is gone and the trimaran is in ruins. Disconsolate, Helen kisses the Mariner in affection and, this time, he reciprocates. While looking below deck for anything of use, the Mariner comes across some of Enola's drawings which include trees. He looks through his own collection including some National Geographic magazines and discovers trees identical to Enola's drawings on the cover. He realizes that dry land may really exist.
Come morning, they are found by Gregor in his hot air balloon and a band of atoll survivors. The Mariner says that he will go and rescue Enola. He takes a craft and finds the oil tanker, secretly boarding it. He kills anyone he comes across while the Deacon prepares a crowd on deck for his speech. He presents to them Enola, saying that with her, dry land will not only be their destination, it is their destiny. The crazed crew then releases massive oars out of the side of the tanker to make their way and to preserve oil - not knowing the Deacon has yet to decipher the map on Enola's back. He keeps her prisoner with Nord watching over her, though she is convinced the Mariner will save her.
Sure enough, the Mariner appears topside and holds out a flare, demanding that Enola be released. He lights the flare and holds it over a pipe leading directly to the oil storage. Deacon calls him on his bluff but the Mariner lets go. Seeing the light, the oil reader utters 'Oh, thank God' before the supply goes up in flames, resulting in an explosion. In the ensuing chaos, the Deacon takes Enola and attempts to escape with her on his plane but the Mariner stops him with an old anchor cable and rescues Enola. Above them, Gregor appears in his balloon with the other survivors and lets down a rope for the Mariner to grab hold. The Deacon rushes forward and grabs the rope too, but the Mariner kicks him off into the water. The Deacon finds a loose jet ski and uses his gun to shoot at the balloon. The shot knocks Enola off into the water and Deacon gestures to two other jet ski Smokers to converge on the girl. The Mariner fashions a makeshift bungee cord and jumps down to rescue Enola just before the Deacon and the other Smokers collide, their jet skis exploding in a ball of fire. As the tanker sinks, the survivors see the words Exxon Valdez printed on the stern.
Then, as Gregor once more inspects the map on Enola's back he has an epiphany and realizes what the markings mean. After a few days of flying in the direction of dry land, the Mariner one morning sees a seagull perched on the craft. A few more hours reveals land in the distance. They land and discover tropical trees, a high mountain peak, fresh water, and even wild horses. A shack is found with two skeletons lying on a bed. On a desk is a music box and as Enola opens it, she hears a tune she's hummed before and realizes she's home. The Enforcer suggests they follow ancient burial traditions and give Enola's parents final rest.
While everyone is thrilled to have found dry land at last, the Mariner finds himself uncomfortable and out of place. The ocean is his home and he decides to resupply, build a new ship, and leave. Enola is upset but understands and gives the Mariner her music box as a keepsake. Before he leaves, the Mariner promises to tell others about dry land and lead them to it. As he sails away, Helen and Enola climb to the highest hill to watch him and come across a plaque that reveals they're standing on the peak of Mt. Everest.