B'ski see the pic. I would put the gate at the red line. Deep enough that one will have to wade to get around it and shallow enough that if one falls they can stand up. I assume the water will be about 2 ft deep there. On the rock - if you are talking about the marked area ( R ) then don't add them there unless very small (no bigger than golf ball size). I would use sand if you want a nice bottom. You don't want someone falling or jumping off the deck on to large rocks. Other places would be better for the SMB as well.