THE PLT RPRTD THE ENG LOST PWR 'WITHOUT WARNING' AS HE & HIS PAX WERE FLYING OVER WATER NEAR THE SHORE AT APRX 500'. HE PERFORMED EMERG PROCEDURES & REGAINED PARTIAL PWR, BUT NOT ENOUGH TO CONT FLT. HE MANEUVERED TOWARD THE SHORE, THEN ELECTED TO DITCH THE ACFT TO AVOID OBSTRUCTIONS & PSNL ALONG THE BEACH. AS THE ACFT NEARED THE WATER, THE PAX GRABBED THE YOKE & PULLED BACK. THE ACFT THEN STALLED & DOVE INTO THE WATER. THE PAX CONFIRMED THAT SHE HAD 'HELD ON' THE YOKE JUST BEFORE IMPACT. THE PAX & SVRL WITNESSES RPRTD THE ACFT WAS FLYING LEVEL AT ABOUT 50' (INSTEAD OF 500) WHEN THE ENG LOST PWR. WITNESSES WHO RESCUED THE PLT & PAX RPRTD THEY OBSERVED FUEL FLOATING ON THE WATER WHEN THEY ARRIVED AT THE ACFT BY BOAT AFTER THE ACDNT. BEFORE AN INVESTIGATOR ARRIVED, THE OPERATOR DISASSEMBLED THE ENG & PLACED THE COMPONENTS IN DIESEL OIL TO PREVENT CORROSION. AN EXAM OF THE DISASSEMBLED ENG REVEALED NO EVIDENCE OF A PREEXISTING MECHANICAL MALFUNCTION OR FAILURE. THE TEMP & DEW POINT WERE 89 & 69 DEG. ICING PROBABILITY CHARTS SHOWED CARB ICE PSBL.