The biggest concern I have with the submerged shelf is that the boulders are placed on top of the plaster. The PB said I could place the boulders directly on the plaster However I did not think this was a good idea so I made a pad for each Boulder with some high strength non shrink grout. So my issue is if the plaster is what makes the pool water tight and plaster does eventually fail, how will I ever replace the plaster that is now under the boulders...
That is one question you might want ta ask the PB.
My fall back plan now is if I ever do develop a leak on the Boulder shelf then I will hopefully be able to seal the front and bottoms of the boulders to the new plaster so water can no longer get beyond the front face of the boulders...

My fall back plan now is if I ever do develop a leak on the Boulder shelf then I will hopefully be able to seal the front and bottoms of the boulders to the new plaster so water can no longer get beyond the front face of the boulders...