I always thought the water rides were open when the park opened at 10. If you don't want to wait long, ride them in the early morning or late at night when it's still a little bit chilly out. The minute that sun comes out and warms things up, you're looking at a 1-2 hour wait, depending on the temperature and crowd size. Plus, if you ride them early in the morning, you'll have a much easier time drying off throughout the day with the help of the sun. If you ride late afternoon or evening, you're more likely to go home a wet customer. I'm speaking from experience on this one. ;)
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and remember, if you can't be an athlete, be an athletic supporter