The pilot/operator reported that he was giving a prospective flight instructor an employment checkout ride. The flight instructor entered a practice autorotation, from approximately 700 to 800 feet mean sea level, to the ocean shoreline (beach) with the plan to perform a power recovery. When the flight instructor attempted to increase power, the power recovered to only approximately 70 percent. The pilot took the flight controls and performed an emergency autorotation to the sandy beach. The helicopter's forward motion was between 3 to 5 knots when it touched down. The pilot said the wet sand was like "glue" and the helicopter stopped suddenly, causing the aircraft to "rock forward and back." The backward motion permitted a main rotor blade to slice through and separate the tail boom. A post accident engine run was performed with no operational difficulties noted. At the time of the accident, the temperature was 48 degrees Fahrenheit and the dew point was 43 degrees Fahrenheit. The carburetor icing curve chart placed the conditions in serious icing for cruise or climb power. The aircraft was equipped with a carburetor temperature gauge and the pilot reported that carburetor heat was on and the needle was below the yellow band. He reported that the time from power off to the attempted recovery was between approximately 15 to 20 seconds.