What makes Adventure Time so great is that it doesn't care. Any given episode could be a Litch-ridden trippy fever dream as easily as it could be a slow-paced kids-friendly B-Mo episode or just another random funny Finn and Jake adventure. This show has no rules and does what it wants, episode after episode, and that's exactly why we all love it so much.
This episode isn't a showstopper, and nothing crazy happens in it. It's just a simple, fun little story about Finn and Jake chilling at the waterpark, and I think that is perfectly fine.
I gave this episode a 10, not because it's jaw-dropping cinema, but because it doesn't deserve the hate it gets and isn't worthy of such a low score. To lambast the shows creators for 'Water Park Prank' goes against the whole laid back vibe of the show, and unfairly criticizes them for mixing things up a little and having some fun with it.
Chill out guys, it's just Adventure Time - not every episode has to change your life.