Problem is I think a lot of people who play more rushdown-oriented characters think that 50-50 is the TOTALITY of the game, and Aquaman is overrated because he doesn't have anything nasty in that department.
And let's be clear, crazy 50-50s ARE a big part of this game, helps make it exciting...but they're not the WHOLE story. Every time you get that 50-50 to begin with, it's because you made a good decision at NEUTRAL first. You got that hit into the combo that sets up your 50-50 because you took a calculated risk at neutral (which may have been very large or very small) and it paid off. Aquaman, of course, has a great neutral game. He may not get the greatest offensive momentum off his reads, but he has an almost unparalleled ability to KEEP the game at neutral where he's usually even or better. That ability even extends to many combos that he's stuck in. Basically the only characters in this game that give AM trouble don't give him trouble because they block well, but because they have great setups AND they give him trouble at neutral. (See: Killer Frost. Frost has (or at least had, when I last played before MMH/Zatanna) a lot of Water-of-Life-resistant resets, but those were all backed up by the threat she posed at neutral to him. Slide was hard for Aquaman to bait and punish and FTD was off the table except as a MAJOR whiff punish because her tracking move was way faster.)
So if you see things purely in terms of paper stats ("oh his overheads/crossups are all seeable, oh he has no standing resets with massive +frames, what's he gonna do, f1 and throw me?"), yeah he kinda sucks from that perspective. But if you think more in terms of player matchups, where people make errors that Aquaman can capitalize on, he's got a lot to work with. I wouldn't call his strength "overwhelming" so much as "extremely well grounded", which I think is a valid approach for HIM in this game, if not for many other characters.
I guess the key to getting down with Aquaman is having a good sense of when you need to play reactively (whiff punish, anti-air, let the other guy move first and WAIT for a mistake) and when you need to FORCE an error by pushing people around a little (f1, b12~DF1 MB, tick throws, the occasional silly Water Shield string). You gotta kinda get down with the idea that advantage in a match is more than just your frame advantage on hit/block or having their face in the dirt. Some people I guess just have more imagination for how to do that with a minimalist character like Aquaman.
In the abstract of theory fighter, everyone blocks/techs/doesn't hit unwise buttons in unsafe situations, and Aquaman sucks. In reality, everybody fucks up or can somehow be MADE to fuck up, and Aquaman has both the punishment and mixup-breaker tools to clean up on those fuckups. Probability is usually on his side. He IS a player's character. An Aquaman player's character, anyway...
