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. 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.
For many years, the pool was not seen, though in the Strong Bad Email other days we learn that Strong Sad and Strong Bad had swim practice on Saturday mornings. This pool 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", but then states that it was imaginary. 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.