Following World War II, Sub-Mariner spent years suffering from amnesia until a chance encounter with the Human Torch restored his memory (Fantastic Four #4).
But suppose the two heroes hadn't met that fateful day. In short, What If … Sub-Mariner Never Regained His Memory?
A back-up story in What If? #29 peered into an alternate reality where a bearded outsider remained ever-unaware of his true identity as the Prince of Atlantis. Answering to the name of "Smith," he joined a seafaring expedition en route toward the North Pole. Taking to heart the writings if Jules Verne and other authors, the ship's captain believed the crew would find an entrance to the Earth's core.
As an aside, Defenders (Volume 4) speculated that Namor's father was in fact Captain Nemo, a character created by Jules Verne.
A series of complications in What If? #29 finally left "Smith" isolated in the Arctic, where his half-Atlantean physiology protected him from the cold. There, he discovered a block of ice that held Captain America in suspended animation (loosely mirroring events from Avengers #4). But in this alternate reality that block of ice never thawed, so the Star-Spangled Avenger remained frozen indefinitely.