I have a natural lagoon pool that I am designing for a client and my desire is to build into the beam natural boulders. In the end I want it to look like the boulders were there and the pool was built around it. I am concerned with engineering and waterproofing. I imagine the shell will have to be built around the stone and then mortar mix would fill the gap between the shell and boulder. Once smooth the interior finish would cover over the mortar fill and blend right into the edge of the protruding stone. I'm trying to describe this as best as possible but don't know if I'm helping or not. Either way I need some input and would greatly appreciate some expert advice.