Yeah, fish people in the lore gives world staff a reason to build underwater things. This underwater update brings a lot of cool building options, and I'd hate to see Massive not take full advantage by removing their fish people before we get them.
As for dealing with the evolutionary physiology thing, isn't the proficiency system meant to remedy the kind of abuse you're talking about? Maybe give Maiar some sort of modular racial bonus, 10 extra points to distribute how they see fit (or maybe limited to particular proficiencies), which reflects their unique adaptation. Alternatively, just have a list of adaptations and let each Maiar pick some predetermined number. Keep them to mostly utility and cosmetic things, which would be a lot more interesting than just another burly race for winning strength contests.
It wouldn't have to be a 10 sentence paragraph for each thing, just short and sweet like a tabletop RPG option:
[Spoiler=Adaptation Examples]
"Bioluminescence: You have some spots that glow with the light of a candle. You can turn them on or off with 30 seconds of concentration"
"Deep Vision: You can see clearly in anything short of total darkness"
"Muck-Swimmer: You can exist comfortable in dirty water such as sewage or that found in the slum canals. You don't risk getting sick when in such water"
"Bone-Crusher: You have a strong jaw and teeth, your bite is strong enough to break human arm bones"
"Electrolocation: When fully submerged you can electrically detect objects within 6 feet of you. You can discern living things from non-living things, and can learn to identify the unique electrical signature of a creature or type of creature."
"Electrogenesis: Once per hour, you can produce a powerful electric shock. A human-sized target is rendered immobile for 6 seconds. Larger targets are stunned for proportionally less time, smaller targets for proportionally more. If above water, you must be touching your target. If you and the target are both submerged in water, they must be within 6 feet of you"
"Lungfish: You can comfortably exist out of water for up to 24 hours instead of the usual 1" [/Spoiler]