In the current game, elves are a species that magically evolve to better suit the environmental niche they settle in . . . so sea elves evolved to breath underwater, avariel evolved wings . . .Even the Sea Elves and the Sky Elves (Avariel) look like cultures, teaching children how to cast a "Gills" cantrip and a "Wings" cantrip, respectively.
If the Sea Elf really is separate, I would rather merge it with the species Nixie.
I've toyed with the idea of creating an elf "trait" called "Environmental Adaptation" with options for breathing underwater, having wings . . . if Mom and Dad were both avariel elves, but for some reason you grew up in the underwater sea elven coral city, you'd be born with gills instead of wings!