In the 40's, the Sub-Mariner was really popular, enough to warrant multiple stories in different books (including Marvel Mystery where he was a regular feature), as well as his own book, which ran for 9 years (till the super-hero implosion in 1949).  When he was brought back into circulation, he had a good initial run of about 10 years or so, starting with the regular feature in Tales to Astonish (with prior appearances in FF).

Aquaman had a solid run in the 1960's in his solo book - he was also in regular back-up features in DC starting sometime in the 1940's, and was one of the only secondary features to survive the super-hero implosion of the late 40s/early 50s, though he never had a solo book during that time period (unlike Subby).  As Sam has indicated, both Namor and Aquaman have had series during the 1970's to the present.

Maybe water-based heroes are too limited given that readers don't live in that environment and can't relate to it.  I can't help but chuckle when aspects of how Aquaman functions was discussed by Raj and the rest in an episode of "The Big Bang Theory" last season... "Where does Aquaman's poop go when he's living underwater?"  Now that could be the basis for an entertaining comic series!

Water based characters, in my opinion to make them interesting, have to be water manipulators as well!  If you came up with a character like Aquaman/Namor and combined them with a character like Katara from the Avatar series, I feel that would be an impressive character, at least from a powers standpoint.  I remember that in the Justice League cartoon show, Aquaman use to hurl water balls.  I don't think I've ever seen him do that in the comic book.  If he could do that on land with other variations of water manipulation powers, I think that comics reader could get behind a water based character. 

Also, and this maybe somewhat sexist, I think females would make better water based characters.  Marina and Fathom were good examples of a female water based  characters, in my opinion.

I loved JB's Namor. It was really well done. But I've never taken to Aquaman at all. The most thought I've put into Aquaman in recent years is based on SpongeBob's Mermaid Man and Barnacle Boy (i.e., I haven't really taken him seriously). :(

I know Aquaman's gone through a few revamps. But one idea that has never been explored (to my knowledge) is the idea of Aqua-man as a Navy Seal: AquaGuard.

I think this could be a cool way to kick-start the DC King of Atlantis, so long as they don’t get lost in the “gritty” side of the Navy. He could be introduced with some really interesting technological devices alongside his aquatic powers (so to be more versatile on land). And his ability to telepathically communicate with the ocean life, as well as being able swim the depths of the ocean, would make him the perfect espionage agent for the Navy.

I love Namor. His presence in the Stern/ Buscema Avengers was great and JB's Namor run was fantastic.

I like Aquaman in theory but haven't found too many good stories about him. One big exception is the way JB and Stern handled him in their JLA story a few years ago. Powerful, confident,a real superhero, not the joke some people make him out to be.  

Still don't care for Aquaman and Namor I like.

Aqualad, both versions, is more interesting than the adult.