As a former dune buggy owner living on the soft sand of Lake MI, that's cool. Have to agree no one will let that on any beach. I am using the soft wheel Cat Trax on a H-18 and going down to the water is easy. With the water low 200 yards and uphill 50 feet isn't fun. If someone could come up wiith something inespensive I'll bet they could make a lot of money at a few regattas.
How about a gearbox from a commercial walk behind lawnmower attached to a 12 volt motor and some ATV tires. All State Parks and most city parks do not allow motorized anything on the beach.