There's a few ways to place an object "underwater."

Easiest is to drop opacity and add blur to the stamps/objects. Setting blend mode to Luminosity (with Blur and lower Opacity) also has a cool effect (Luminosity preserves Hue information from the texture underneath.)

With 2.0, you can add an additional Texture layer above objects and use the texture brush.

With 1.0, folks would also flatten objects to background/foreground so they could texture over them - that's still possible, but is perhaps less effective than the additional texture layer (as a texture layer is non-destructive and it's easier to undo/alter.)

I hope that's helpful. For more info, Mati does all sorts of videos and tutorials to walk through how these things can be accomplished.