The swimming pool was used by Leela, and Borusa also hid from the Sontarans in this area. Both the Fourth Doctor and Leela referred to this room as a "bathroom", and the Doctor described what K9 called "[being] totally immersed in H2O" in this room as a "fine time to take a bath". (TV: The Invasion of Time)
He later restored the pool, and used it to brew bootleg beer in order to infiltrate 1929 Chicago's gangster subculture. Ace failed to appreciate the state of the pool when she wanted to swim, nor did she appreciate the Doctor's joke that swimming in the beer would "give it added body." (PROSE: Blood Harvest)
When Izzy Sinclair had her body temporarily swapped with that of the alien Destrii, Izzy had to occasionally use the Eighth Doctor's swimming pool to keep Destrii's amphibious body alive. (COMIC: Beautiful Freak)
When the Tenth Doctor and K9 visited a planet for several hours to help sort out an issue on a planet of slobbering dog monsters, Rose-the-cat became very bored and began doing random things in the TARDIS. One of these involved visiting the TARDIS pool and throwing potted planted into it.
Sometime later, when the Doctor was attempting to form a list of things to clean in the ship, the pair discovered the plant-polluted-pool. Rose acted just as shocked as the Doctor was, pretending to be wholly unaware how such a thing could happen. (COMIC: A Rose by Any Other Name)
When he first met Amy Pond near Easter 1996, the Eleventh Doctor was soaked because — as he explained to her — he had just climbed out of the pool which had fallen into the TARDIS' library during the crash. After the TARDIS finished repairing itself, the Doctor mentioned that he wasn't sure where the swimming pool was now and suggested that it was possibly in the wardrobe. (TV: The Eleventh Hour)
The Doctor took Alice Obiefune to the pool during her first visit on the TARDIS. She mistook several creatures swimming in the water for goldfish and the Doctor explained that they were actually "a highly aquatic alien race whose world was destroyed," one of whom was named Reg. Later, Alice witnessed the Kharitite and its owner, a giant adolescent walking squid, playing together in the pool. (COMIC: After Life)
The Doctor landed the TARDIS against a partially constructed skyscraper in July 1969, with its exterior oriented face-up. He then instructed Amy to open the doors all the way to the swimming pool. A moment later, River Song dove from several stories above into the pool, breaking her fall. Her impact was sufficient that some water splashed even beyond the exterior doors. This was done to fake River's death in the eyes of the Silence. (TV: Day of the Moon)