Our cobble-boulder seawall on Canandaigua Lake (NY) has many gaps where mortar is missing due to weathering and wave pounding. I'm seeking advice on how to repair the wall. Specifically:1) preparing the rock surfaces (I assume muriatic acid and a stiff brush to remove slime/moss, etc. would be good); 2) mortaring joints below the water -- is there any way to do it? I have learned of the compound Antihydro which makes the mortar water repellant. Any other tips?