Rangers do fairly well in wilderness which water is.
A staff that heals plus decent tanking makes them pretty effective underwater. Net casting is a good way to safeguard a sole entry point to, which is the case to actually lots of underwater areas (aboleths, ship graveyard, coral palace, coral head, ruins of the deep all have single entry points)
In fairness, I only ever adventured with one, a long time ago, but he seemed pretty good at ranking in the ruins of the deep, and he had a pendant that healed him underwater (that I have never been able to find with any character- I only know the name of the mob it is on because of locate object, but never found the mob which was a sahuagin)
At the same time, I believe I fought a nexus one with Salyeris a few times on the Sea of Despair, and generally beat him down pretty easily.
For water exploration though, I'd say mariner rangers are up there only behind water shapeshifters.