Before commencing a fishing expedition, the pilot had determined that a particular section of beach was frequently used by light aircraft. An uneventful landing was made on the beach, and later the pilot made a take-off and circuit of the area before landing on another section of the same beach. The group had no success with their fishing, and the pilot decided to fly to another beach on the opposite side of the bight. During the landing roll, the pilot discovered that the left brake was not operating. The aircraft subsequently ran through a shallow water run, entered an area of soft sand, and overturned. The pilot had no previous experience in operations from beaches, and the operator of the aircraft was not aware that a beach landing was intended. The section selected was not used by other pilots who operated in the area. The reason for the brake failure was not determined, however the left brake unit had a recent history of malfunctions, possibly related to defective seals. This accident was not the subject of an on scene investigation.