Q:  What did the water say to the boat?

A:  Nothing, it just waved.

Explanation:  “Wave” is what you do when you move your hand as if saying hello.  “Wave” is also the name for a way the top of the water moves: the raised part of the water that moves toward the shore is a wave.  This joke plays on the word “wave” and is funny because the water waves (as if saying hello) while the water has waves (the raised part of the water).