Also a way to kill Aqua:
When he goes into the water/puddle form where he can just zoom around, when he goes back to the solid being form he’s ok but if he went into another body of water in his puddle form, when he transforms back water would be on either side of his skin, even the inside and could get into his lungs, bloodstream, etc. but no, aqua can not drown, he’s an axolotl. But I’m pretty sure that water in your bloodstream may not be super great-
Also just put him in a bucket in water form. He’s trapped. He can’t go up steep surfaces. Also if he tries to turn back he would just be stabbed by a bucket.