There once was a time when various comics companies would just toss random characters out there and see what stuck. While the Silver Age version of characters like the Flash and Green Lantern first appeared in DC Comics’ Showcase, that particular comic was initially intended as an anthology to introduce new characters. In point of fact, the first character to be featured in Showcase was a firefighter named Fireman Farrell. He got three short stories in that issue and as near as I can make out was never seen again.
As a digression, Farrell’s last story featured a national news TV crew following Farrell and his company around as they fought a fire, but that was to cover a story about a local (and apparently unpopular) ballot initiative to give the firemen a raise so they could do stuff like send their kids to camp (that was treated as a tragedy). One woman interviewed said the teachers deserved a raise first, and a home viewer decried that woman as “stupid”. I sure would like to know why wanting a raise for a nation’s educators is stupid. Or why a local ballot initiative was national news. I guess the point is I don’t miss Fireman Farrell.
But that “let’s see what works” approach is my best explanation for the Sea Devils.
The Sea Devils weren’t actual devils. Nope, they were deep-sea scuba divers. That’s all. No special powers or skills beyond being good swimmers. Somehow, their frequent trips to the ocean floor got them into all kinds of scrapes. Maybe it was aliens, or undersea pirates, or giant octopi, or mythological giants, or whatever. It was on the bottom of the sea, and the Sea Devils had to deal with them. My guess is Aquaman was busy.

Hey, the oceans make up something like 75% of the world’s surface. You can’t expect one guy to be everywhere there’s a problem.
That said, there were originally four Sea Devils. First appearing in August of 1960’s Showcase #27, their leader was all-around smart guy Dane Dorrance. He was probably the most well-adjusted of the bunch. Then there was his blonde girlfriend Judy Walton, who was glad to go diving because under the sea there was no discrimination for her being a woman and she could be the equal of any man (though Silver Age stories tended to make sure that wasn’t quite true). The third was Judy’s younger brother Nicky, who enjoyed diving because there was no discrimination for his being a teenager and he could be the equal of any adult (I’m sensing a pattern). Finally, there was a big guy named Biff Bailey, who enjoyed being under the sea because he was clumsy on land but could be graceful underwater.
Huh. A double-initialed leader, his blonde girlfriend, her kid brother, and a big guy friend of theirs…that group sounds familiar…

The Sea Devils went on the sort of crazy adventures the Silver Age used to have all the time. They didn’t have to make sense. The series ran for about 35 issues.

And it’s not like the group just vanished without a trace. The Sea Devils, or sometimes just Dane and Judy, continued to appear here and there. They got some international members with a single name that were probably unintentionally racist, and even today the Sea Devils (minus Biff) are popping up in the New 52 as, er, eco-terrorists.
But DC had all kinds of similar characters who just went exploring in places and get used as necessary in their superhero universe. Characters like Rip Hunter, Cave Carson, and the Challengers of the Unknown just went off on crazy adventures despite usually having only one skill that most anyone could theoretically pick up (you know, except for Rip Hunter’s time travel thing). The average underage reader didn’t know there weren’t giant, hostile living statues on the ocean’s floor, so why not? What set the Sea Devils apart was they could hang out with the Little Mermaid.
Oh, and they weren’t Fireman Farrell.
