The man known only as "Admiral Poseidon" was the Baron in charge of King's Point Naval Station. He and his close friend Bronson found the place and set about rebuilding it; over the course of time, Poseidon's madness came to the fore, and he became obsessed with power. He gathered an army around him, both enlisted men and mercenaries, and discovered a working Los Angeles-class nuclear sub, armed with a single Tomahawk missile that lacked a warhead.
When one of the mines he'd set in the ocean off the Georgia coast sank the boat Ryan and his companions were on, resulting in the loss of Krysty and Jak, Ryan swore vengeance. He got onto the base and into the sub as Poseidon took it on its maiden voyage. It was intended to be a short cruise to test-fire the missile, but the sub hit a mine and sank with Poseidon on board, while Ryan escaped.
Poseidon was a huge man - 6'6" and around 300 pounds, all of it muscle. He has graying hair, a thick beard, and eyes that shade from gray to black depending on his mood. Rumours have it that he was the result of genetic manipulation; he even let slip that he "grew up behind glass, with my father and mother observing me... (like) one of their own freakish experiments". (Watersleep)
Note: The final page of Watersleep noted that Poseidon was still alive on the ocean floor in the remains of the sub (and that he has some ability to breathe water, probably through gills), but he has not made an appearance since.