what could happen if you drink too much water right after exercising?

Hyponatremia or water intoxication but you'd have to consume a gallon or so. Water intoxication can be fatal.

Usually 20oz of water per hour of exercise is plenty.

you could drop the levels of salt in your body to a dangerous level. thats y gatorade and stuff is good cause it has small amounts of sodium for this reason...if you can take the amount of calories it has. if not add a tiny tiny big of salt to youre water

you just throw up, nothing bad jeez