Seahorses would make sense
A chariot pulled by seahorses for a chariot UU....
I think a sea based production resource is needed. Perhaps just "Minerals" like in alpha centaurii.
I'd suggest having Kelp as a feature, which gives +1 food. And a Deep Mine improvement that they could build anywhere, and which gives +1

(Not +2 like normal mines) Kelp should be something which you can build stuff in without removing it, and it should also give some sort of movement/strength penalty to naval units, allowing it to be used as minor defence against those surface dwellers
Some other possible features...
Geothermal Vent: Gives a significant amount of commerce on the tile
Coral I think, could give a small amount of both

and
In general though, I would advise to keep sources of

relatively small and not easy to get. So that this civ would be somewhat industrially hampered, as a tradeoff for the vast amount of commerce and food they're going to have.