"The Slytherin Common room is obviously underground, but what you probably don't know is that it's under the Black Lake. It gives an interesting ambiance, but it also reminds you that under the calm water you see at the surface, there are many things swimming around all at once that can distract you if you let it. Oceans can occasionally have a riptide, such an extreme undertow that you can't help but get knocked out of your normal place. If you fight directly against it, like some people do, you'll tire yourself out swimming nowhere as the shore line disappears as the water carries you out to sea.