If you stay at a rental property off-site are there any problems getting into the waterpark at peak times?

No problem at all.

Are there any special going on??

There is ample car parking and it wasn't very packed on the day we went which was a Saturday. It is an awesome place to go.... We loved it...

Parking is free by the hotel. They don't check the cars. Keep in mind they want your business. So enjoy!