He takes Watson's hand, knowing full well they are doing something very stupid, but he can't quite be bothered to care right now. It will be stupid and different and risky and interesting, and it will be infinitely better than not doing it. Unless they become sick, in which case he'll just have to blame Watson.

"That is easily promised," he says, squeezing Watson's hand and smiling a little too much for someone standing in the cold about to get even colder.

There's no way around this; going in slowly will only result in backing down from the challenge, so he starts them at a run into the water, which is shockingly, numbingly cold, but he can only start laughing at the ridiculousness of this.