Immortal Kombat Water Shield doesn't make Aquaman "completely" safe on block. Technically, he's NEGATIVE on block from Water Shield ender even if he drops it immediately due to the recovery time on dropped Water Shield. The main utility for Water Shield string ender is if you're doing something that's negative on block anyway (say b12) and you want to see if the opponent is dumb enough to throw an unsafe string at Water Shield...or if they freeze up seeing Water Shield thinking "oh fuck armor, don't hit buttons", you either backdash if you think they're smart enough to poke a forward dash cancel, or just dash up and throw a la an SF4 FADC if you think they're gonna just sit still and block. It's not useless, but it's DEFINITELY not something you can do in EVERY situation where a move gets blocked.
It's really 17 frames to gain armor on Water Shield? I thought for sure it was closer to 10. No wonder wakeup Water Shield sucks so bad. But yeah, a lot of people (who don't play Aquaman) don't realize that he doesn't gain the armor immediately upon startup. If he did, shit would be way too crazy. Deathstroke guns would be nearly useless against him, lol (YOU CAN'T SHOOT ME BECAUSE I'M WET, BITCH).
This move definitely has a lot of applications, but it's not totally free like a lot of people assume it is. It has punish application against slow stuff that's advantageous on block (Doomsday MB shoulder), but just sitting around with Water Shield up in most matchups waiting for something to happen is dangerous. You generally need good reads AND good spacing/frame advantage in order to use it as a counter-poke (I guess more like an anti-poke?), because the effective startup (really hate that the game lists it as 0f startup, VERY misleading) makes it tricky to react with on short notice.
Punishes from Water Shield release are also harder than they look because the recovery is not instant when you release the shield. Scorpion teleport MB punishes, for example, are surprisingly tight. You have to release the shield basically as soon as you eat the 1st hit in order to recover fast enough to punish Scorpion with b12. Hence why I suggest using it only for stuff that inflicts a ton of blockstun but otherwise has a lot of recovery such as DD MB shoulder or b3s/f3s.
Using it against a character like Superman IMO requires a bad opponent more than anything else. Using Water Shield against advantageous stuff with long reach but SLOW startup is quite smart, but a decent Superman has both the mobility to keep you from backing out to safe Water Shield activation range and of course his f2 has a stupid speed:
ange::safety:
eward on hit ratio to begin with. He basically has to intentionally let you out to use Water Shield, unless you guess right at round start with it, in which case you may never need to use it again if you can keep your momentum.
tl;dr this move is good but it's not THAT good.