"The Saltwater Wife" has its origins in the selkie legends, wherein certain seals could turn into beautiful women. Men could 'catch' these inhumanly beautiful creatures by stealing their sealskins, and thereby trapping them in their (did I mention incredibly beautiful?) human forms.
I wanted to tell a story in which some of these women retain their agency, but I also wanted to tell a story about being human. This isn't a story about some battle of the sexes; it's a story about personas. In my experience, we are all pretending on some level. I'm not saying that it's a bad thing, just that it gets complicated. I don't like to tell stories about Strong Female Characters. I like to tell stories about human nature. Even if my characters aren't always, you know, technically human. Thus, selkies and shifting identities were in. Sexy fish-ladies were right out.
One final note: In reality, eels and groupers do hunt together. They're one of the few common inter-species hunting pairs found in nature. The eel swims into tiny crevices to flush out prey; the grouper snaps up the victim, leaving some nibbles for the eel. They're an insanely efficient hunting team. Is this a metaphor? Perhaps.