in 2009 i attended a massive which took place in a huge hotel with a pool/hottub area. although there were no slides, rafts or rides, the feeling of thizzing and the warm water on my skin was amazing coupled w/ the lights & dj's in the pool area. so many people and so much fun. it was an absolute hotel takeover! great memories of that weekend. i'm sure alot of midwest heads will forever remember that one!
i'm sure a waterpark would be a blast too - but it's just not the same when everyone around you isn't rolling too - the air lacks that special magic. wouldn't mind giving it a shot someday, though!
i'd say MDMA for the waterpark, but reserve the MDA for just wading in a pool and/or hottub.