How waterproof is that stuff? I'm putting in a stream and there are many
wide flat limestone rocks that I would like to incorporate, while using EPDM
liner for most of the stream. In particular, there is a rock about five
feet in diameter, about 4 inches thick that has a nice overhang. I would
like to dig the dirt back about a foot, lay liner back under the rock and
foam the seam where liner meets rock so I can have the stream flow over the
rock and fall to the pool below.

Will the foam be good enough to waterproof the seam so I can have standing
water there, or only enough to deflect splashing water with the water level
lower than the edge of the liner? Is there a better way to do this? I know
I can do the Gatorgard spray-in stuff, but that's on the pricey side of
things.