WHILE EN ROUTE, THE ACFT'S RADIO TRANSMITTER BECAME INOP; HOWEVER, THE PLT COULD RECEIVE TRANSMISSIONS & WAS PROVIDED VECTORS TOWARD THE DESTINATION ARPT. AFTER STARTING A DESCENT NEAR THE DESTINATION, THE ENG LOST POWER. SUBSEQUENTLY, THE ACFT WAS DITCHED IN THE TENNESSEE RIVER. A WATER LAND ENG WAS CHOSEN BECAUSE OF THE ROUGH TERRAINS & DARKNESS. AFTER THE WATER LANDING, ALL 4 OCCUPANTS EXITED THE ACFT WHILE IT WAS STILL FLOATING. ACCORDING TO THE PLT, NONE RECEIVED INJURIES DURING THE FORCED LANDING. ONE OF THE PASSENGERS COULD NOT SWIM. SHE & ONE OTHER PASSENGER WERE PRESUMED TO HAVE DROWNED. AFTER THE ACFT WAS RECOVERED, ONLY RIVER WATER WAS FOUND IN THE FUEL TANKS. THE ACFT HAD FLOWN ABOUT 4 HRS & 6 MIN PRIOR TO CRASHING. FUEL CONSUMPTION FOR THE FLT AS ESTIMATED TO BE 60.27 GAL. THE FUEL CAPACITY FOR THE ACFT WAS 60 GAL. IT WAS NOTED THAT THE NON-INSTRUMENT RATED PLT WAS BRIEFED TO EXPECT IMC CONDITIONS ALONG THE ROUTE OF FLT. DURING THE FLT, HE HAD CRUISED AT 8000 FT. AT CHATTANOOGA, THE CEILING WAS 3000 FT.