Oh, there are more. But those are the only two that aren't completely lame. I mean, the Human Flying Fish? The Fisherman? Aquabeast? Torpedo-Man? The Bugala? Most of those didn't even appear more than once or twice.
Then there are the good villains, like Charybdis and Kordax, created by Peter David... Charybdis was turned into a joke by a pathetic later writer. Kordax was apparently killed. Most of the decent ones don't get re-used. Instead writers go back and grab either Black Manta or Ocean Master.
I guess that's why I'm so disappointed with the return of Ocean Master. Can't we get somebody ELSE for a change? Go back into Aquaman history and find somebody interesting? There are hundreds of loose ends a good writer could take advantage of... why don't they ever? Is it editorial fiat or ignorance?
Hey now, don't you go dissin' the Fisherman. He was LEGO before even Black "I am wearing two umbrellas for a head" Manta, and well before your johnny-come-lately Aquaman. We Fans of the Fisherman may be few, but what we lack in numbers we more than make up for in passion. (And poisson.)
As for Manta not being a threat, if being the first villain to kill a family member doesn't give you street cred, than nothing will. But it is not the character's fault. (His "People of Race are more successful out of the Surface World" schtick is also very powerful, but I will be the first to agree that it is too high concept to ever really sell.)