Doomsday Scavenger: Ocean King

Eugene digs the beach on a small island. Supplies come up out of the sand for him, and the sea animals that come near the shore will do what he tells them. He got hold of this after the water took the land. A tsunami went over the continents and never went back down, and what is left above the surface now is islands like his. Other survivors are scattered over the water. He brings them in a few at a time until the island holds a group of them, and the outfits that want the sea to themselves come after what he has. Then a merfolk civilization comes up from the deep water, and some of the help he has been getting turns out to have come from them. While he is working out terms the two sides could live under, the system puts a warning in front of him: the person he trusts most is carrying the curse mark of the merfolk royal line.