First Appearance: Daring Mystery Comics #7 (April 1941).
Golden Age Appearances: Daring Mystery Comics #7-8, Comedy Comics #9.
Modern Appearances: Avengers #97, Invaders #5, 4 #1
Years Active: 1941-?
Lieutenant Peter Noble of the United States Navy is at sea, part of the crew of an American submarine, when there's an accident and the sub sinks. He's forced to swim from the sub but discovers that he's somehow immune to the pressures of the deeps. (Which might make him a mutant, or perhaps the accident activated his powers Godlike-style.)
Peter goes out swimming and discovers the underwater kingdom of Neptunia. After meeting the natives and adventuring among them he becomes the ruler of Neptunia, gaining a magic cutlass and the ability to swim underwater without using any breathing equipment. (When he'd first left his sub he'd used an oxygen tank.)
In the modern era he was one of several Golden Age heroes conjured up by Rick Jones during the Kree-Skrull War in Avengers #97. In Invaders #5 Namor the Sub-Mariner is shown a picture of the Fin; Namor's response is to sneer at it.
The Fin has superstrength, can breathe underwater and survive the pressures of the ocean floor, and is quite good with his superhard, supersharp magic cutlass.
Note: Namor's response to the Fin is ungracious--but this did happen in 1941, when Namor was even more hostile than he usually is. And, better still, the Invaders reference brings the Fin into modern continuity.