I'm thinking of untying the boat tied next to mine while the occupants are sleeping inside. What's your thought?
That seems pretty extreme. Why do you want to untie their boat?
It sounds like a fun prank to pull on them.
Untying someone's boat while they're asleep inside it is really dangerous and you shouldn't do it. Pranks are supposed to be fun, and aren't supposed to injure people or damage their property. If you untie their boat then you could get into serious trouble.
Why not?
Just saying that unseen things can unfortunately happen, like strong tides or waves that can sweep them away. It may seem funny to you, but wouldn't you think there are better ways to prank them? I'm sure you can think of something!