In preparation for a local revenue sightseeing flight with two passengers, the pilot performed a preflight inspection of the airplane and used a generic fuel quantity dipstick that required converting the observed quantity to an actual quantity using a conversion chart. The pilot stated that he inadvertently arrived at an inaccurate quantity; instead of the 12 gallons of fuel he believed was on-board, the airplane likely only had 6 to 7 gallons. About 20 minutes into the scheduled 30-minute flight, the engine lost power completely, and the pilot subsequently performed a forced landing to a nearby beach. During the landing roll the nose landing gear collapsed resulting in substantial damage to an engine mount tube. As part of the recovery from the beach, less than 1.5 gallons of fuel were drained from both of the airplane’s intact fuel tanks. A postaccident check of each fuel quantity indicating system revealed each gauge indicated empty with the fuel sending unit fully extended.