Devil's Advocate

I think that the best way to make a true waterborne species would be to look at how they would evolve in a watery environment. You're right in thinking that this species would just have fins, unless they're amphibious. As a result of this, this species would have a hard time developing any sort of technology (except for maybe building shelters, as some modern-day species do). However, if this species is crustacean, they could perhaps develop advanced technology because they could build with their many appendages. Since a crustacean-based species would most likely not be bipedal, their technology would be designed to fit their type of body and any vehicles would have propellers and fins. As for abilities, you can go pretty much anywhere with it. You could take inspiration from the Mantis Shrimp, which can see ultraviolet light and give a deafening punch.

I hope this helps, I see a lot of potential in this.
The Tin Woodsman