

Your words "a
rather aquatic race" sets up an expectation in the viewer.
Most of this expectation is implied as no other information is provided.
The common thought that comes to mind is that this guy lives underwater.
Anatomy design has many factors.
An important one is mobility.
Creatures use their adapted bodies to move thru their environment.
Certain forms and features are better suited to specific environments with specific types of mobility.
Of course an adapted body is not necessary for specific types of mobility with the power of technology.
People cant fly so they use airplanes.
People cant run 70mph so they use cars.
From this characters body and features I imagine he would need and be able to develop similar technologies to get around underwater.
Visual theme carries a lot of power (and because art is visual this is important).
Its not necessary at all to design an aquatic character to have extreme fish like features.
However it makes him quickly readable, relates to the natural world, and distinguishes him from characters of other enviroments/classes.
Of course character design isnt nature or science.
Its enough to just make something cool, whether it fits how we relate it to the natural world or not.
Telling the viewer what something is has a lot of power.
Just keep in mind that without telling us anything, would we all come to an agreement of what this guy is, where he lives, etc?
An easier description to swallow might have been "a race that uses water magic".
Two great pieces of advice that have always stuck with me:
1. "Show me, don't tell me"
2. "Always error on the side of more extreme"
The idea being: try stuff, and if it turns out really silly, it's easy to remove.
Pushing a design on paper can reveal appealing options that seem ridiculous in the mind.
But I'd say dump all of my design choices here and come up with your own.
It's obvious that you are going for something different than a fish man.