Crap, you're right. Ex-nay on the upidstay.
Merfolk don't suffer penalties outside of water - except that they can't walk, since they have a big giant tail fin instead of legs. And while 5e may not force them to breath via gills, a lot of settings do. "The Little Mermaid" certainly did.
In any case, wild elves and winged elves (and star elves, unless those are supposed to be Eladrin in the 5e Realms) aren't represented yet in D&D yet either. Let's not single out just Sea Elves, and instead be consistent. The reason Half-Sea Elves got to join was because they've been a much more consistentpart of the game.
You can still play all those (except Elderin for some reason), in AL, you just use the rules for the other Elven subraces instead, which makes sense for Star Elves (basically another high elf subrace) and Wild Elves (related to Wood Elves), but absolutely insane for aquidic, lythari, and winged elves.