In a recent interview, David Tennant revealed that an "inappropriate" underwater scene was cut from an episode of Season 3. Freema Agyeman, aka Martha Jones, was required to strip down to her underwear for the scene, which was later cut by the producers.

"There was an underwater scene in a draft of the new series but we nixed it out," explained Tennant. "It was bordering on inappropriate. You can't have shagging in the Tardis."
hehe