In contrast, Bill Everett's Sub-Mariner-- who debuted alongside the Torch in the same 1939 issue of MARVEL COMICS-- always seemed to have a lot of unused potential, whether the character was handled by his creator or by latter-day talents.
For one thing, the Sub-Mariner (a,k.a. "Prince Namor") has a great visual design. With his triangularar head and pointed ears, he seems to me like a less hirsute version of the Greek god Pan.
And that's not even taking into account the ingenious (if improbable) attribution of his power of flight to the tiny wings on his heels, obviously derivative from Hermes/Mercury's winged sandals.
The Sub-Mariner's origin-story is extremely promising, pitting the youthful hero against the surface world due to an attack on his people, also called "sub-mariners" (and not called "Atlanteans" until the 1960s reboot).
However, with a real World War threatening to engulf America, there was no possibility that the Sub-Mariner's storyline could have continued to focus on a war between sub-mariners and surface dwellers. Namor quickly dropped his grievances against Americans and became a crusader against the Nazi menace for most of his Golden Age career. Yet his best-remembered stories are those when he was still somewhat on the side of the devils, as when he battled the not-so-human protector of surface dwellers, the Human Torch.
Indeed, one of these encounters even involved Namor hurling a tidal wave against New York City, though he repented of his warlike acts and went back to being a good guy, without anyone raising a stink about the death and destruction he had caused.
The 1960s reboot heightened Namor's role as a decent guy embittered by the way nuclear testing had dispersed the people of his Atlantean empire. (Later continuity rewrote this story to exculpate Americans once again.) Namor was most frequently opposed to the Fantastic Four, though he was made more sympathetic due to his rather goatish lust for Sue "Invisible Girl" Storm. The character didn't get his own series until 1965, in TALES TO ASTONISH #70. Unfortunately, Stan Lee and Gene Colan got off to a poor start with a soggy quest-story that couldn't compete with similar tales from Lee and Kirby in the THOR title.
Things looked up somewhat when Sub-Mariner got his own title, written for several years by Roy Thomas.For a time, artist John Buscema also gave the series an epic feel somewhat reminiscent of Hal Foster's PRINCE VALIANT. However, no single story or set of stories used the hero to best effect. The closest Namor came to glory during his own magazine's run was when Thomas and Buscema had him encounter the denizens of another sunken city, Lemuria. This was the most ambitious serial in the history of the Silver Age comic, but though it introduced the artifact known as "the Serpent Crown"-- a major trinket in Marvel continuity-- the Lemurian tale lacked cohesion. The character's mythos also suffered from a mediocre set of villains who failed to challenge the hero on a deeper conceptual level, as the Fantastic Four's foes did for that group.
For the most part, later series starring Namor also failed to use his mythic potential. However, I finally re-read one particular stand-alone story that meets my mythcomic criteria-- and interestingly enough, it's by Namor's creator Bill Everett, who was allowed to write and draw several issues of the Silver Age title the artist passed away. The story even manages to play into the element usually neglected in Namor stories: i.e., his relationship to the Greek deities, as well as commenting on the character's intrinsic connection to humankind's practice of the art of war. More on this story anon.
CORRECTION (8/29/18): I credited the Lemuria sequence (SUB-MARINER #9-13) to artist John Busceme. Evidently I was only remembering the big Sub-Mariner/Thing battle that precedes the sequence proper, in issue #8, for the artists in #9-13 are Marie Severin (issues 9, 12 and 13) and Gene Colan (issues 10 and 11).