Superheroes from Marvel and DC Comics are often military veterans, but one of the core branches is conspicuously absent: the United States Navy. Writers from both companies have often placed their major heroes in the armed forces to show their determination, tenacity, and courage under fire, and this practice hasn't fallen out of favor, even long after the Golden Age of comics has passed. With so many comic characters in the Army, Marines, and Air Force, this naturally begs the question: why are there no superheroes in the Navy?
Though the Golden Age of Comics arguably began with the debut of Superman in 1938's Action Comics #1, the genre truly became popular at the height of World War II. A slew of patriotic heroes emerged, most notably Captain America, whose entire origin story centered around the super-soldier serum that allowed him to join the United States Army. In the 60s and 70s, more heroes followed in Captain America's wake: Green Lantern and the Punisher were United States Marines, and War Machine, Captain Marvel, and the Falcon were all active-duty members of the Air Force. But outside of rarely-seen D-list characters like Deadzone (with a grand total of only five appearances, all in the 90s), no major Marvel or DC superheroes have ever served in the United States Navy - and the reasoning is ultimately a tragic one.
From a Doylist perspective, it's simply more exciting for a character to journey into unfamiliar territory; the writer can create new locations and new enemies to challenge the heroes. Characters in the Army, Marines and Air Force could march/fly toward a new area and find themselves in dangerous places, perhaps even behind enemy lines. While a character in the Navy could technically experience the same situation, they would be limited to the sea; one unknown spot in the ocean looks much the same as any other, and as comics are primarily a visual medium, there's only so many ways for an artist to draw a vast body of water. But in all likelihood, this hypothetical Navy hero wouldn't get many chances to see the ocean anyway.
The vast majority of sailors on military ships carry out their duties belowdecks; even on modern ships and fantastically large aircraft carriers, a sailor can - quite literally - work for days or even weeks without seeing the sun. Thus, there are plenty of World War II accounts of sailors feeling quite helpless in battle, as many never even saw their enemies during combat. Usually, their only opportunity to see the aggressor would be after the battle, either to pick up survivors if they won...or to abandon ship if they did not. This is the inherent irony of conflict at sea: for all the expanse of the world's oceans, one's world is limited to a small, enclosed ship.
The above situation doesn't prevent a hero from being a Navy veteran, however; perhaps their experiences on ships would help them in their post-military career as a potential Avenger, Justice League member, etc. The hero could also be a naval aviator or even a Navy SEAL; their training would absolutely help them as a superhero. With so many Marvel and DC heroes in the Army, Marines and Air Force, it's odd that the Navy is without comic book heroes to call its own. However, with the current influx of new characters in comics, perhaps it's only a matter of time.