(Uplifted dolphin. Yes that is an official race)
Bwahahaha! OMG, why have I never played Traveller?!
So unless you plan to make a rather defining feature of that race irrelevant by offering easy, never breaking tech solutions everywhere I stand by my judgment that a purely aquatic race is not playable in mixed groups.
I feel ya, and would hesitate to incorporate them in a fantasy setting, but would counter that any interplanetary sci-fi setting requires readily-available and largely reliable tech solutions just to happen at all - environment suits, air supplies, radiation proofing, very fast vehicles, energy sources, communications, etc. If a setting includes androids, cyborgs, and robots/AIs, these characters ARE tech, and must be assumed to be easy and never-breaking. I don't think it unreasonable to have an aquatic race be tech-dependent for exploration/integration, since everyone else in such a setting will be as well, to one degree or other. Even if every planet the PCs visit is "class M" and every PC is an oxygen-breathing carbon-based life form, they can't get there and back again or face down bizarre alien threats without a lot of awesome tech they can count on.
That said, and given Morrus's notion that this species are particularly skilled at medicine, I'd think a permanent or semi-permanent biologic solution would be more suitable. If their science is "magical" enough, they may even have it both ways - perhaps they can create a new racial ability to reconfigure their tails to become 2-4 limbs that can "zip" together to reform the swimming tail (with a penalty to swim speed compared with non-augmented members of their race). Weirder yet would be lengthening and strengthening their tails a bit to be more snake/eel-like, so they can slither rather than walk, or coil the tail like a spring and leap, which would still allow them to swim to a degree. You know, similar to yuan-ti or Medusa from
Clash of the Titans. This requires the science/tech up front, but would be largely one-and-done, leaving the PC free from worrying about long-term gear.
And with THAT said, were I to run a sci-fi game with non-human PCs, I'd be looking for ways to highlight and exploit such specific limitations and weaknesses to present character-driven challenges for them to overcome. If a player wanted an aquatic race, I'd be keen to point out where things could go wrong, and probably make them happen if the player started taking things too much for granted or didn't seem to be playing the character as "fish out of water".