That's a nice cover image; the middle bit would look great as a book cover, too. :D I've already added this fic and its predecessor to my marked for later list on AO3, looking forward to them.

Btw maybe you've seen the fanart of Riku and Sora as mermen? It only just occurred to me that the fanart's main idea is a bit like Omegaverse for mermen, though idk how this would work in terms of anatomy. ^^;;