Well, I'm on my phone right now so please excuse any grammatical or common errors.
The main and most important reason in which Shazam beats Aquaman is that Shazam absolutely destroys Aquaman on wakeup. Everyone knows just how terrible Aquaman's wakeups are, even KTP went on a little rant on how Aquaman can't get off the ground. Shazam has been surprisingly blessed in this MU because he has a plethora of options to butt-fuck Aquaman on wakeup. Lemme list some things that he can do:
CMD grab on wakeup
Read wakeup and punish after blocking it.
Allow wakeup to whiff and punish it.
Cross up and can do j3/j2 into CMD grab which is tremendously fucked up in this MU.
In short: Shazam can mind-fuck the Aquaman player on wakeup because we have so many more options to continue the hard knockdown vortex.
Now that I finished discussing about wakeups, I can go into several other reasons on why shazam beats Aquaman.
1) Shazam's movement is one of the best in the game, and he has no trouble getting in against Aquaman.
2) Shazam is one of the few characters that can reliably punish Trident rush.
3) Shazam's b2 and CMD Grab have excellent range which allow's him to bypass some of Aquaman's footsies that the rest of the cast find hard to deal with.
4) Whiff punishing with b2 and CMD grab is also easy so the aquaman player can't be pressing buttons the whole time.
5) B2 and CMD grab can open the Aquaman player up, since one is an overhead that goes half screen and the other is command grab that has very good range, is very fast, and leads to a full combo. If the aquaman player knows this, he/she is obviously gonna start being jittery and pressing buttons and that's where point "4" comes in, you can make a smart read on when ever their gonna press buttons and slowly walk back, and when they whiff whatever their doing, you can whiff punish.
6) Teleport means Shazam can get off the ground free and judging by the situation, can either get in or run away from aquaman. It's essentially a free Forward/Back dash on wakeup and it's ALMOST unpunishable.
In short: Shazam is allowed to play his game against Aquaman, but Aquaman can't play his game to the fullest against Shazam.
I like arguments, so if you have any please provide them.
EGP Wonder_Chef