The boat course guy hangs around the cafe bar in Windfall at night. There, he says you'd better be careful about throwing bait in the water, because the mermen might come up and pull you under water. Initially, I figured this referred to the painter fish that do your map, but the more I read it, the more it seems to be a referrence to the possibility of Zora in the Great Sea.
As for Rito, according to the story, they develop wings when Valoo gives them a scale (sort of like how Zora scales let you swim in OoT) when they come of age. I don't know where they come from or anything, or if they're Zoras, but that's what it says. . .
If you ever do any of the Nintendo Gallery, be sure to get the two "Rito" dudes at the flight platform. . . rather odd.