The_unbeliever's post in Jet Ski Mishap was marked as the answer
Well, I usually set an anchor so my boat doesn't go adrift... Huh, I'm nitpicking, ain't I? >_>
In fact, I was wondering whether you meant a logical problem in the guise of a simple geometrical one (with the rope length missing on purpose).
And yes, I think I should keep the maths to pen & paper - I'm obviously missing ^2s here :/ My bad.