"What do you mean you can't swim?!"
Sherlock shrugged, his shoulders pressing into the back of the couch. When John continued to splutter at him he chucked the remote petulantly.
"It's not as if it's a useful skill," he huffed, waving away John's screech of disagreement. "For a consulting detective, John. London's my place of operation. What, is a criminal mastermind going to throw me into the Thames?"
"This gets out? They just might." John said. "What a terribly easy, embarrassing way to get rid of Sherlock Holmes." Sherlock rolled his eyes.
"Please. You'd jump in after me."
"I wouldn't."
"You would."
"Well regardless you need to learn." John insisted.
"I did learn. Mummy's fault."
"… and then you deleted it."
"Brilliant deduction."
"You're learning again."
"I'm not."
"You are."
Had John bothered to consult Mycroft about this he would have learned of such an endeavor's futility (as well as some horrifying childhood stories. Pity that.). Turns out there was a reason Sherlock had deleted swimming lessons.
As it was, John dragged Sherlock to the public pool. Three hours later—with skinned knees, bruising to his left elbow, tufts of hair missing, a floatie stuck around his neck, and possibly developing pneumonia—John lead a miserable Sherlock back to the flat.
He was a genius, yes. He was not buoyant.