According to Jim Recap: The Swimming Pool
It's one hot summer, and it's made even hotter by the fact that the air conditioner is broken and Jim can't fix it (and he's too cheap to pay to get it fixed properly). When he tries to fix it by taking parts from the one that Andy has - working - at his house, he ends up breaking that one too. But there is a solution, it seems. The neighbors are gone, and they have this lovely swimming pool just sitting there not being used. So why not take advantage of it?
Cheryl. It seems that she has this problem with them using the pool. She's supposed to be collecting the mail, and she doesn't want the neighbors to think that she is taking advantage of them by using the pool while they are gone. So Jim agrees. But when Cheryl goes next door the next day, she finds Jim and Andy there in the pool - along with everyone else. Even Sherri (Andy's girlfriend, the lady from the storage place in Trashed) is there. Cheryl kicks them all out and tells them to clean up.
So they reluctantly get out and tell her that it's like they were never even there. But when Cheryl goes to check the mail, she sees that they left it a complete mess, so being the good person she is, she starts to clean up. That sun beating down on her, however, starts a process in motion that she can't deny. And soon, she's in the pool, and it's like an addiction. She's sneaking around to get in a swim - she even kills a blow-up pool toy so that she doesn't get found out, which is when Jim catches her.
That's when Jim gets turned on, and so they go for one last swim before the neighbors come home, and they decide to leave the suits outside the pool. Which might have been a mistake, because the neighbors come home while they are still in the pool.
