Now here's the real reason I think Aquaman has to stay on his toes in this match. DS bullies him pretty hard on HKD. Here's a little trick I'd like people to look into:
When you get a DS combo on Aquaman, end your combo with D1 xx Flip OR 2 xx Flip. After the Flip hits, do 1 dash, and do F3. This beats ALL of his wakeups (barring the super of course). If you try this in training mode, you can set the CPU to "Random" wakeups. It doesn't matter, this F3 will beat every wakeup they try to do, you don't even have to guess.
Of course, Aquaman can block it, but F3 is +10 on block, allowing you to do B1 or B2 for a follow up mixup.
Aquaman CAN backdash this, but of course, there are ways to punish the backdash if you know he will be doing it (such as a Sword Flip for a HKD or Machine Gun for an easy 11%).
Basically, you can keep Aquaman ensnared in a constant 50/50 guessing game from landing just one combo. This is pretty significant.
On top of this, Aquaman is pretty free to jump attack pressure when he gets HKD. Water shield can get him out of this stuff though.
You want to be either full screen or right in Aquaman's face in this matchup. Midscreen is a no-no. Jumping in the neutral game is suicidal.