"shoot simulated movement of river water"
ah, second time read it and now it could be, that i know what u want xD
Dou you want to create splashes on the water surface, caused by bullets?
For this the easiest way would be:
Testing, if a bullet touches water (in_passable eg.) and then create a sprite which looks like a blue ring. This ring grows, if u increase the scale_x and scale_y of this entity every frame. Decrease the alpha during the growing and it will look like a little wave.