Just wanted to say first off that I love this game.
But man are those piranha's nasty. The only way I've ever found to deal with the room is the potion of inivisibility, and I know I have had to go back a level with one I found lower down; so if that's the intended "solution" either it's not a guaranteed one or I just really stupidly did my potion identification mid-level. (in general I ID scrolls at the top of a stairs and then potions after I go down)
Handy tip though: Even if you're invisible, count the piranha in the room closely, then look at the chest REALLY closely. I learned the hard way that a piranha sleeping under the chest will mean you hit it, thus losing your invisibility, instead of opening the chest. Being left all the way across the room from the door, even with three healing potions in hand, was a purely lethal experience.
If anyone has found a better solution to the flooded vault, speak up.
-abs