Same exact thing happened to me when I took the boat to a friends "rental" house in Stone Harbor (approx 30 minutes south of Atlantic City) except I was at the rental house floating dock. When I arrived/docked I had 9'-6" of water underneath me. Went to dinner and when we came back (at dead-low tide) the boat was on the bottom i.e., 6" of water. What I didn't realize what that it was a full moon which instead of a 6'-6" tidal difference there was a 9'-0". The only saving grace was that A. No cement block were underneath me and just as important, since my drives were down, the drives were in what must have been the edge of the channel i.e., in approx 3' of water.
I stayed in the water for 3 hours cleaning the bottom, drives and anything I could to keep my mind off of hearing crunching fiberglass. Thankfully it never happened. Ah boating, there is nothing like it.