Fluid Effects > Ocean > Add Dynamic Buoy
 
 
 

With a single command, you can add a NURBS sphere (a buoy) to your ocean and make it bob up and down in the water. The motion is restricted to the Y direction.

Fluid Effects > Ocean > Add Dynamic Buoy >

If you want to move the position of the buoy interactively, the buoy cannot be connected directly to the expression. In this case, turn on Free Transform.