We have DQ-WP annual passes so going for the end of the day was viable for us. We just got back from two weeks there and went to the water parks four times (3 times BB, 1 time TL), each time during the last two hours of the day.
The locker rentals are just a buck (net of the $2 refund) at that time and the lines for the slides thin out. No, it doesn't give you time to do everything but you can park pretty much at the front of the park (since the folks who were lining up at 9am are heading out right after lunch).
The first two times we went were rainy days and EVERYTHING was a walk-on. The last two were hot, summer days and the crowds were there, though thinning out with every passing moment (and, no, we never got shut out of the park).
Just as a tip, if you go when the park opens, hit the slides first then the lazy river and wave pool once the crowds build. If you get there later in the day, do the lazy river and wave pool first to give the slide lines time to clear out.