Given a depthmap, you can find all the stuff that's shallower than some percentage using this workflow snippet.

In the yellow box, enter a percentage.  The mask will mask everything shallower than that %age - so if you enter 0 it'll mask nothing; if you enter 1 it'll mask everything, if you enter 0.5 it'll mask the "foreground" ie everything that's 50% deep or shallower.  Any number from 0 to 1 will work.