We did G09 last year. No touching bottom, the water is 10 to 20 feet deep. The snorkeling is amazing, though, so don't be put off. Our kids had never snorkeled at all (11, 9 & 9 when we went) and we had to drag the twins out. They were literally the last ones out there. We had fully mature stingrays plus some amazing coral and incredibly clear water. The float vest makes the entire evolution much easier and the crew of the catamaran had 2 in lifeguard/over watch while the third person was in the water with us luring the rays in with squid and the like.
I completely understand for non-swimmers to do the sand bar. If you and they are competent swimmers G09 was a blast.
Whatever you choose, have a wonderful cruise!!