This is from a couple travel books, plus what does make sense too. Stay away on week-ends, year round. People usually arrive on a week-end and stay the week. First they hit the parks and then finally get to the waterparks, DisneyQuest etc. SO, what's recommended in several books and what I do, is usually go to these things on Monday, Tuesday or Wednesday. I still get there upon opening to avoid lines for Shark Reef and the slides. We usually only stay about 1/2 day too.
But i've never gone to the Waterparks off season. We've usually only gone to WDW in early December or the summer. Although last July when we were there, I was on the lazy river and one of the CM's were counting people and said it was the least crowded day she's ever seen!!! Go figure!!!