how are radio waves used to control a boat’s direction?

It’s a solar boat it’s made of styrofoam with an electric motor

You need a chip to interface with the motor. You also need a receiver on board the boat. Basically you have to control the motor’s speed and the rudder. For that you can get a motor out of a toy plane.

The above link might help

You can build a better boat if you can program the chip. It can then chart a pre configured path.