Unconsciousness from a balloon are quite temporary and shallow, since as soon the inhaling stops normal oxygen levels begin to saturate the bloodstream. Most videos of people fainting from balloons are kids or smaller individuals, for adults several breaths of inert gas are required to lower the oxygen so low you pass out.
My guess is that as soon you start breath water, the body will fight for survival and snap out of the shallow unconsciousness. But if you're are a particular small or frail individual, it's not impossible. People drown in public swimming pools all the time for various reasons. I wouldn't count on it through.