Aquaman was pissing me off. I went into training mode, found out some stuff
Trident thrust attack:
Jumping is a little risky because of how the trident thrust hit boxes work (some are angled and act as anti airs). Blocking it leaves him at advantage (puts you in stagger state), so you want to stay just outside of it's reach (roughly 3 character spaces away) and AR for max damage should he whiff it.
From the depths (ground poke):
This is a right tricky bastard to deal with. It tracks for ages; beats AR (takes too long to start up); trades with gun shot (both variations) in his favour; 8% on hit (2% on chip) and causes knockdown (all of Aquaman's knockdowns set him up for anothert 'From the depths' special). Fortunately, you can avoid it completely by instant air gun shot, causing 2% and building meter at the same time. If he has knocked you down with it, crouch block to avoid the next one.
Trident throw:
By itself it's not a problem, since you can low gun shot, jump,sword flip etc on reaction (and not lose any health in the process). The problem lies after he knocks you down with this - it allows him to set up from the depths.
Sea shield (or w.e the hell it's called)
He has to hold the button down for it to cover him and continue for the duration of the move - he can't do anything else whilst it's up. Dash in, throw his bitch-ass and reset spacing game.
Aquaman has a lot of overheads with extremely good reach. It's in your best interest to keep him outside of trident thrust range, and just react to whatever he does accordingly. If he does ground poke, IAGS and bait him on wake up. If he whiffs trident thrust, AR. Trident throw, low gunshot/sword flip on reaction.