Swimming

Swimming was the act of propelling one's self through water, typically on its surface. When practised competitively on Earth, it was often done in an Olympic-sized pool. Competitive swimming events were often over distances greater than a single length of the pool, requiring swimmers to be able to "flip" efficiently at the end of each length. (PROSE: Father Time)

However, swimming was sometimes done purely recreationally, as Ace did on at least one occasion in the TARDIS' Olympic swimming pool. (PROSE: Loving the Alien)

Swimmers

Miranda was a particularly good swimmer for her age -- indeed, she threatened at least one Olympic record -- in part because of her flips. (PROSE: Father Time)

Perhaps unsurprisingly, the Doctor, too, was a proficient swimmer in many of his incarnations. (TV: Warriors of the Deep, PROSE: The Bodysnatchers, The Devil Goblins from Neptune) In his second incarnation he told Serena that he had taken his Gallifrey Lifesaver's Certificate at the Academy. (PROSE: World Game)

Turlough was a good enough swimmer to be able to rescue Peri Brown from apparent drowning in the ocean waters off Lanzarote. (TV: Planet of Fire)

Melanie Bush was a fan of swimming and was swayed to visit Paradise Towers mainly on the strength of its sales pitch about its pool facilities. Unfortunately, her attempt at swimming there was sabotaged by aquatic robots. (TV: Paradise Towers)

Non-swimmers

Peri Brown was not a particularly adroit swimmer, as evidenced by her need to be rescued by Turlough. (TV: Planet of Fire)