Kevin Costner commits suicide at the end of WATERWORLD
Kevin Costner had been, up until the end of the film, the future of humanity. As an ichthyus sapien, his mutation (gills behind the ears) showed that humanity was slowly (perhaps too slowly) evolving to deal with the deluge they brought on themselves. The Atoll way of life is dying (this point is made explicitly clear at several points in the film). Humanity scrambled to save itself as the waters rose, but it's been hundreds of years and the supplies are now rusting to nothing. Food is scarce, and so is fresh water. Who knows how many more generations can survive on the open water? Probably not many. Like Helen says, they were not meant to live on water: "we have feet, and hands". Humanity could endure, but not survive indefinitely under the conditions of Waterworld. Perhaps Costner represented a kind of future, one that may have held promise had evolution continued. The Mariner himself was not sufficiently evolved to live without the Atoll/barter system. He could not survive in the open ocean. He needed fresh water. He couldn't even hunt the giant fish that roam Waterworld without left over dynamite. What happens when that is gone? Starvation. Kim Coates as the paper merchant, shows us that the situation in Waterworld is getting worse, and has been getting that way within Costner's lifetime. Coates had a fishing pole that he was proud of and refused to trade. When Coster kills him and throws the pole overboard, Helen protests: Coster just shakes his head "There's nothing you can catch with that". He doesn't say it, but it's clear what the end of that sentence was: "Any more". Perhaps there were fish to be caught at one time, but the remnants of humanity fished the oceans bare just like their ancestors had been doing before the poles melted
When The Mariner discovers dry land at the end, he knows that his branch of the evolutionary tree just became obsolete. Humanity now can -- and will-- survive without gills. The atoll life is still dying. That has not changed. Those left on the ocean will slowly die out, and the inhabitants of dry land have it in their interest to keep the terrestrial surface of the earth a secret. Costner is going back out to sea with supplies, that may last him a week, a month, a year... but ultimately he's setting course for a way of life that has no future, knowing that he himself has no future.
So, in that sense, it's not the mariner going home to the sea because he can't deal with life on dry land, it's the mariner doing what old Lakota elders used to do.. wander off into the winter to die of exposure, as to not be a burden on the living.
PS I also do not think he had male genitals. This is just a pet theory of mine, but given his awkward interactions with Jeanne Tripplehorn I tend to suspect he was not capable of breeding with homosapiens. He's kind of like an aquatic mule that way.