Right now - and this can change by the time I'm done writing this, the best day for a water park would be Sunday. And that forecast says "isolated thunderstorms developing throughout the afternoon". But that's for all of Kissimmee, not right at a water park. And "developing" could mean anything and it's a 30% chance.
Now, check tomorrow and everything will have changed.
Heh - I just checked today. Now only a 20% chance of rain on Sunday and nothing about thunderstorms.
We needed them a couple of times last year this time, but normally the rain didn't last long. We had to hop out of the pool at BLT one afternoon due to lightening in the area, but still plenty of fun to be had indoors.