All depends how hot you like it to use water parks, We used to go October, perfect weather but the last few days started to change and cool down. Xmas this year was a no chance, warm for a few hours middle of day but took a while to warm up and then cooled down late afternoon. Easter this year was too cold 1st week, great the 2nd week.
You can get good days at anytime of year but for constantly hot days I would not go beyond Oct or Before May, but we love August 100+ temps, others prefer spring/Autumn weather.
So to answer you question May or Oct I would think are the best times for you for good weather, not too hot, cheap rates and low crowds.