In this post-apocalyptic world, resources are scarce and the remaining humans have turned to a nomadic lifestyle where they live on boats that have been adapted for survival on the open ocean. The protagonist, known only as "The Mariner," is a mutant with gills and webbed feet who is hated by other humans due to their fear and prejudice against mutants.
When The Mariner arrives at a floating trading post, the barmaid, named Helen, catches his eye. However, their encounter is interrupted by an attack from pirates. The Mariner saves Helen and her charge Enola, who has a mysterious tattoo on her back that The Mariner believes will lead them to dry land.
As they sail away from the trading post, they are pursued by both the pirates and a group of "Smokers," who are a violent faction that uses technology salvaged from the old world. Along the way, The Mariner and Helen grow closer, and he teaches her and Enola how to survive in this new world.
Eventually, they find a mythical place called "Dryland," but they must overcome obstacles and face off against the pirates and smokers who are determined to capture Enola and claim the prize of Dryland for themselves. In the end, The Mariner sacrifices himself to ensure Helen and Enola make it to Dryland, where they discover a garden and signs of a new beginning for humanity.