Underwater Stones: There’s no need for your river rocks to feel so all alone
-First off we need to copy our rubble layer by either dragging it down to the new layer button on the layer tab or going to layer⇨Duplicate layer with it selected.
-move it beneath the foam layer but over the water layer.
-Ctr+a to select the whole thing
-select the layer mask
-either use the paint bucket to fill the layer mask with black, or hit delete with black and your background color.
-now select white as your foreground color
-select your pebble brush
-draw some stones in your river.
-now you’ll change the opacity and the fill. Fill affects the visibility of elements that aren’t layer effects, whereas opacity affects everything. as light and shadow show better in water than other traits we’ll lower the fill as well as the opacity. Here’s the settings I used:
Click image for larger version. 

Name:	Tutb1.jpg 
Views:	417 
Size:	237.5 KB 
ID:	63925
The same technique could be used for anything else you want to submerge, bodies, boats you name it. For deeper objects you could use some filter⇨Distort⇨ripple to show distortion of the image coming through the water.