User Guide > Dynamics and Effects > Fluid Effects > Fluid Effects Menus > Fluid Effects > Fluid Effects > Pond >
Fluid Effects > Pond > Add Dynamic Buoy
With a single command, you can add a NURBS sphere (a buoy) to your pond and make it bob up and down in the water. The motion is restricted to the Y direction.
Fluid Effects > Pond >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.