Waterworld (1995)

The polar ice caps have melted, and the earth is covered by water. The Mariner lives aboard his triple-hulled catamaran. Mariner, self-sufficient and solitary, is an outcast because he has developed functional gills and webbed feet. Looking for supplies, Mariner visits an atoll fortress where the people, frightened and half-crazed, imprison him in a cage and intend to execute him.