We went mid_March in 2004 and the weather was fine for swimming and water parks. The day we went to the water park it was overcast and sprinked a couple of times which helped keep the crowds low, but was comfortably warm and didn't really rain until that night. We had a great time at the water park.
During our visit we had one rainy day and even then, it stopped by mid afternoon. Kept crowds low at MK. Otherwise the weather was great.
When we went both water parks were open, but one had just recently reopened.