Aquaman as Namor. at least the Brave and the Bold has changed that up (by recasting him as Hercules).
Orm, Aquaman's half brother (known to many of us as The Ocean Master). he of course wants to rule Atlantis and of course, Aquaman doesn't see his treason until it's almost too late.
Deadshot to kill Aquaman, which leads to one of the best Batman bits in the show. he misses several times. spoilers:
we see how Aquaman loses his hand (another of DC's rather dumb ideas, especially replacing it with a three foot trident. well, it changes size on a regular basis.
it's not one of my favorite episodes. it seems like it's just a way to introduce Aquaman to the cartoon, though they did that already in Superman: The Animated Series, though in the more classic form. and he was nicer there, too!