Personally, I don't think you need 2 days. We bought a 2 day pass because it was "only" $20 more than a 1 day pass, but we kind of felt like we wasted our money. If you plan on doing the water park and all the regular rides, then you could use 2 full days. We rode everything we wanted to on the first day (except for one coaster which was down that day). The second day, we rode that coaster and then did the water park. By noon we were ready to leave. (I admit we're not huge water park fans. We did the lazy river and a couple slides and were done.)
If you do the water park, WEAR WATER SHOES! The pavement is scorching hot and my husband burned his feet. There are very few places to escape from the heat at KD. Few restaurants with AC, not a lot of shade. We're roller coaster junkies and went to cross those coasters off our lists, but I doubt we'll be back.
