Fluid Effects > Pond > Add Dynamic Locator
 
 
 

Dynamic locators follow the motion of the pond in the Y direction, but additionally react appropriately to dynamic attributes (in the Extra Attributes section of the Attribute Editor).

Fluid Effects > Pond > Add > Dynamic Locator >

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