Great Stuff to Seal Leaks?

While building a patio next to the pond (2500 gal, Permalon) I uncovered a 4' x
2' x 6" granite rock. Of course I just HAD to incorporate it into my pond's
shoreline. Using a lever and slowly rolling it on pipes I got it into position
and leveled it. It looks great! Unfortunately my pond now has a slow leak and I
am 95% certain it is under this boulder.

I'm thinking of using Great Stuff to fill the leak and I'm wondering if
GreatStuff works underwater. I plan to use a long extension tube to apply it
way back in the deep recesses behind and under the boulder.

Question: Does anyone know if it will be necessary for me to lower the water
level and allow the Permalon Liner to dry completely before applying it or will
GreatStuff adhere to wet areas?

Thanks,
Gary