From Homestar Runner Wiki
Homestar Runner characters can occasionally be found lounging and swimming at the swimming pool. It is an in-ground, Olympic-sized pool with a white and red checkerboard-patterned liner. It appears to be located somewhere in The Field, within walking distance of the Contest Area. It also must be near an electrical outlet, as a radio with a power cord has been seen here. In Strong Bad's Cool Game for Attractive People it is located near the race track on the opposite side from the locker room. It first appeared in A Jumping Jack Contest, when Strong Bad was seen reclining in a pool chair while waiting for Homestar Runner to fail at the jumping jack contest. Later, it appeared in the Theme Song Video, revealing a diving board from which Homestar jumps. The pool also has a higher diving board, which everybody tries to keep Strong Mad off of; due to his mass and his ability to perform a "mannonball" dive, he can drain the entire pool by jumping off the high-dive.
After its debut in A Jumping Jack Contest, the pool was not seen for many years, though in the Strong Bad Email other days Strong Bad claims that he does not have swim practice on Saturday mornings with Strong Sad, and that he definitely does not wear a Speedo, claims which are shown to be blatantly false. The pool finally reappeared in the email keep cool, where Strong Bad asserts that they've always had it and used it all the time. He is also the resident lifeguard. According to him, the pool rules are to keep crap out of the pool (banning The Poopsmith), no rough-horsing, house-rousing, butt-patting, running, diving, or swimming (though the last one is not widely enforced). Strong Bad had apparently placed chemicals that he found under the sink (Bleach Juice, Awesome Sauce, and Roach Killa!) into the pool, causing Marzipan's hair to turn green and burn away. In the event of a lightning storm, all the characters he likes have to leave the pool, and the characters he hates (namely Coach Z, Homestar Runner, and The King of Town) have to enter holding metal objects, such as TV antennae.
In the book Where My Hat Is At?, a different, bean-shaped pool belonging to Homestar was seen. Also, during the In Search of the Yello Dello Commentary Strong Sad relates how "back in school" every one started being really nice to him because he had a pool, which may or may not be the pool in question.
In winter pool Strong Bad states that in the winter they drain the pool and he makes "a buncha cool skate videos that accidentally get recorded over with cooking shows". He then reveals that they filled it with Fluffy Puff Translucent Dessert Related Substance which Homestar received as payment for the commercials he acted in.