There’s a swimming pool aboard the TARDIS. Not some piddling little one either, it’s full size, you could hold the Olympics in there. Donna’s kind of impressed because… Well, indoor swimming pool inside a police telephone box! She doesn’t even try to understand how that works, or how the water doesn’t all slop out when they’re travelling through the vortex.
Slipping off her shoe, she dips her toes in the water; it’s pleasantly warm. She should probably fetch her swimming costume first, but the temptation is too great.
She doesn’t know who’s more embarrassed when the Doctor catches her skinny-dipping!