Fluid Effects > Pond > Float Selected Objects
 
 
 

You can make an object float by adding a dynamic locator to it. Dynamic locators follow the motion of the pond in the Y direction and also react to dynamic attributes such as buoyancy, gravity, and damping.

Fluid Effects >Pond > Float Selected Objects >

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