I wanna talk more about why this is so funny to me.
So as you probably know, Water Park Prank was a guest-animated episode from season six by an animator called David Ferguson. It is widely considered one of the worst episodes in the show because of its basic plot, sometimes gross jokes, and offputting style.
It was following up on two incredibly well-received guest-animated episodes, A Glitch is a Glitch and Food Chain, so it had big boots to fill. And more importantly, it was not even supposed to be an 11-minute episode. Ferguson was originally commissioned to only make a five minute short to be posted on Cartoon Network's website and maybe its YouTube channel. And his style would have worked well in this short format, as it does for his other work. But he ended up accepting a deal to adapt his 5 minute online video, which was already mostly finished from what I understand, into a full episode. The deadlines were pretty unforgiving and the result was basically just the original video (consisting of the actual water park scenes) with a b-plot about the "daddy sad-heads" frankensteined onto the beginning and end.
So it was kind of doomed from the beginning to not be a great episode. Difficult deadlines, a doubling of the runtime late in production, a style that was intended for 2015 YouTube being broadcast on TV, and almost certainly the lowest budget of any of the guest episodes. The fandom hated it, and still mostly does. If you ask anyone for their least favourite episode of Adventure Time, and if they didn't stop watching after Breezy, there is a pretty good chance they will say Water Park Prank.
Now one thing about guest-animated Adventure Time episodes is that they are generally not considered canon. If you go to the wiki page for most of them, you will find a note somewhere saying that they are not canon. But this is not the case for Water Park Prank. Because the day before Water Park Prank aired, main series writer Jack Pendarvis proclaimed on Twitter that Water Park Prank was CANON.
An important thing to note is that there is in fact no established definition of Adventure Time canon. Jack Pendarvis probably made the tweet in full knowledge that it meant nothing but that it would be taken seriously. And taken seriously it was, because fandom is fandom. To this day I believe Water Park Prank is considered canon on the fan wiki. For a long time I considered it canon on my own website.
The net result was the hilarious paradox of this random episode simultaneously being the worst guest episode and also the only canon one.
Things went on like this for a while. Bad Jubies came out and was really good, which secured Water Park Prank's legacy as the bad apple among a bunch of awesome guest episodes. Ferguson sort of accepted his episode's reception. Way later the crew got him back to animate a segment in Distant Lands BMO, perhaps as a jokey sort of redemption, or perhaps because his style was actually pretty cool from the beginning and hadn't exactly been allowed to shine under the previous conditions.
Enter Fionna & Cake, which gives us an in-universe definition of what it means to be canon; a canon measure of canonicity, if you like. A dimension is "canon" if it is connected to the wider web of the Multiverse, at which point it also exists as an asteroid in GOLB's realm. Of course, this is still a different thing from actually being canon to Adventure Time, and there is still no official definition of canon in that sense. But it's close enough.
And what was one of the dimensions that Simon steps into ever-so-briefly in GOLB's realm? Is it the iconic 3D environments from David OReilly's A Glitch is a Glitch? No. Is it the wonderfully colourful natural history museum from Masaaki Yuasa's Food Chain? No. Is it the lovingly crafted clay world of Kirsten Lepore's Bad Jubies? No.
THAT'S RIGHT BABY, IT'S THE FUCKING WATER PARK!!! Canon first by word of god, and now by in-universe definition too!
In honour of this momentous occasion I'm not going to implore anyone to actually watch Water Park Prank. But next time you're doing a full watch-through of the series, maybe, just maybe, consider not skipping it.