Roger Frank Selby 
Catch of the Night [PDF ebook] 
When women evolve faster to the aquatic environment of a new world, drastic mating methods are required

It is the night of Two Moons Passing – An important right-of-passage night fishing trip for Jude, a young boatman who has never seen a woman in his life. His is a simple fishing village with sailboats, but where are the women? The small population comes from boy babies, occasionally, and now very rarely found on the beach. The village is aging, dying. On a waterworld with no continents, these are the sex-segregated descendents of the crew of a crashed starship where the women have genetically adapted faster to the aquatic conditions while the men lag behind. And on such a world, drastic mating methods are required!