I read somewhere that since most people arrive at WDW on Saturday or Sunday, they often hit MK or one of the other parks first. Hit the water parks Monday or Tuesday, the thinking goes, and you'll beat the crowds.
Guess what? We tried that just last month at Blizzard Beach and it was GREAT!! Crowds were minimal and manageable the first two or three hours of the day. (And this was in July!!) By lunch, when we left, it was picking up.
But early in the week is DEFINITELY the time to go to the water parks!!