Dudes of the deep, indeed! Earth-Two's premier protector of the deep was Aquaman (so was Earth-One's), once a lad experimented upon his father utilizing ancient Atlantis techniques to not only help him survive the deep but manifest mighty muscles and manipulate mollusks!
On Earth-X, those crafty Nazi scientists developed the means to mutate a mortal man into Sea-Wolf, a hybrid being himself capable of also residing underseas while displaying a wide-range of physical attributes, including an animal side.
And why do we know that Sea-Wolf threatened the oceans of that world? Well, the fact that the balance of power was tipped in German's favor there instead of America's on every other world, it would seem logical that most of the native meta-humans residing there would support the Nazi regime. And the reappearance of Earth-X as Earth-10 during the Infinite Crisis indicates this a reality.