So you want to pull a cruel prank on your friends while they are swimming? Turning a remote control boat into a shark fin and move it around your friends is one way to do that. CobyUnger has published a guide on Instructables.com that shows how you can build your own RC shark boat.
You are going to need a RC boat, some paint, fiberglass, acrylic sheet, and a few other parts to complete this project. More instructions can be found here.