Block. Block low if that's not working. I can't remember off the top of my head if Aquaman's seismo is low or not. Though you want to block low most of the time against Aquaman anyway thanks to that scoop.
If someone's using the same moves a lot, then bait it out of them, block, and then punish.
If Green Lantern is doing his grab when you get close, try dashing in, immediately blocking, and then attacking after his grab fails.
If he keeps catching you when you jump in, then... well, stop jumping in. Sometimes your strategy just isn't working and you have to try another approach. I play Hawkgirl and I abuse the hell out of her flight, but for some characters or players that just means free hits, so I have to stick to the ground.
As for Aquaman's seismo, you just need to slowly push your way in. Block, dash. Block, dash. Repeat until you get close enough to attack. The same goes for most zoning, like Deathstroke's guns. God help you if you play a good Cyborg, though.
And just a general piece of advice: if you keep having trouble with a specific move, go into training mode with your character and the one you have trouble with, then select Record from the AI options. Record the opponent doing the move and then holding back for a moment afterwards. Now he'll just keep doing that move, and you're free to experiment with different approaches. Try blocking and then countering, for example. If he blocks it (since you held back after doing the move), then it's not a good punish. But if you can hit him, it means you can hit him before he's even able to block. So, you know, do that.
Sorry if it sounds like I'm talking to you like you're an idiot. I don't know your level of experience, and these are the kinds of things I wish people had told me when I was first trying to figure out fighting games.
