Another thing to note if your plans are flexible (this is a change we decide to make):
Monday would definitely be better than Sunday, but unfortunately, this year they are closing each of the water parks two days per week. On Mondays Typhoon Lagoon is closed, which means that BB will be the only water park open. I strongly suspect that means the crowds will be higher on Monday than they would be on Tuesday, Wednesday, or Thursday, since everyone who wants to go to a water park will have only one choice instead of two.
As sunking says, nothing beats getting there early, and it should be a slow period anyway, so this is just an additional consideration to keep in mind.