Yeah, we only know about a handful of aquatic species on Oddworld (Gabbits, Sea Rexes, Scoots, Stangs and Mudflubs), some being ocean-dwelling, some being freshwater, some being amphibeous. But since we haven't actually started exploring water on Oddworld, I'd say we've no reason to have seen that many water-dwelling creatures. MO ended near a river (according to the fabulous Oddworld Map), and those baby Gabbits have to hatch and live somewhere, and I'm sure we're all hankering for Munch-oriented gameplay in OW4 (following the Abe-lead action in MO).
What's more, the monkey/horse-centaur wants to get stuff across the river, right? :P
My guess is that Oddworld's underwater flaura and fauna is just as many times more diverse than ours as Oddworld's land-lubbing critters are. Urg, hope you don't get as confused with that as I did.
Bah!