A good long while ago, I wrote a story of a new crewman to the Enterprise, whose previous assignment was to a Starfleet vessel that was configured for a aquatic species, basically the ship's interior was flooded. You learned of the Try-la through flashbacks.
I could have made the Try-la look like the creatures from the black lagoon, but instead I made them mermaids and mermen. We know from the the example of the Xindi that a Cetacean species can have starships. Just as there are multiple Humanoid species, why not multiple aquatic star-fairing species as well. From dozens of different worlds there would be cetaceans, whales, CBL's, mer-people, and true fish-people too.
What benefit would a aquatic species bring to Starfleet? Looking at Earth as a typical class M world, the majority of the surface is water. The aquatic crewmembers would be assigned to investigate the oceans and waterways of interesting worlds. They would beam into the waters of these worlds, interact with the aquatic lifeforms they found there.
On some aquatic starships, the corridors would be three quarters water with a gaseous atmosphere near the ceiling, whale-people need to breath. On others starships the fluid would posses a higher concentration of oxygen (or other gas) than terrestrial water does, amphibians and intelligent fish-people need to breath.
The Try-la are warm-blooded amphibians, they have legs and not tails. Their fingers and (long) toes are webbed, their forearms and crura have boned fins that extend outward when needed. The Try-la have internal lungs and not gills, the water they live in has more dissolved oxygen than ours.
The new character is Jer'notta (Jehr-NO-tah), Jer'notta entered Starfleet later than most, having already served a decade in the Try-la Interstellar Guard, mostly in anti-piracy patrols. Her assignment is in sciences, she is a talented shuttle pilot and is a security auxiliary.