TARDIS swimming pool

In many of its incarnations, the Doctor's TARDIS had a swimming pool.

The swimming pool was used by Leela, and Borusa also hid from the Sontarans in this area. Leela mistakenly called it "the bathroom," confusing the pool for a bathtub. (TV: The Invasion of Time)

Peri once used liquid nitrogen to freeze the pool into a skating rink, much to the Fifth Doctor's displeasure. (AUDIO: The Roof of the World)

The pool was later jettisoned due to leakage, which Mel found bothersome as she wanted to go swimming at the time. (TV: Paradise Towers)

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 implied that he swam laps in the pool as a way of avoiding Amy and Rory when they were being amorous. (TV: Amy's Choice)

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)

The Eleventh Doctor later burned up the swimming pool to get enough energy to enter the bubble universe. (TV: The Doctor's Wife)

The pool was back in the TARDIS in its next incarnation, and Clara Oswald came across it while trapped inside. (TV: Journey to the Centre of the TARDIS)

Behind the scenes
A longer scene involving the TARDIS swimming pool was cut from The Doctor's Wife because Karen Gillan can't swim.