ACFT COLLIDED WITH THE WATERS OF NARRAGANSETT BAY AT A LOCATION APRX 1 MILE FROM BOYS TOWN ARPT (DESTINATION OF RECORD),IN WATER APRX 8 FT DEEP. THE WX WAS IMC AT DEPARTURE (1845 EDT) AND THE ATP INSTRUMENT RATED PLT HAD REQUESTED AND BEEN GRANTED A SPECIAL VFR CLEARANCE OUT THE CONTROL ZONE TO THE SOUTHEAST. THE CONTROLLER LATER STATED THAT HE AND ANOTHER FAA EMPLOYEE ON DUTY HAD DISCUSSED THE REQUEST FOR SPECIAL CLEARANCE AND HAD EXPRESSED SKEPTICISM AT A SVFR OPERATION IN THE EXISTING WX CONDITIONS. ATTEMPTS BY THE FAA CONTROLLERS TO OBTAIN MORE CURRENT WX AT QUONSET STATE & NEWPORT STATE ARPTS WERE UNSUCCESSFUL. QUONSET TWR DID ACKNOWLEDGE THAT A 1 AND 1/2 MILE VISIBILITY EXISTED AT THE ARPT AT TIME OF ACKNOWLEDGEMENT. THIS INFO WAS PASSED TO THE PLT OF N45397 PRIOR TO DEPARTURE. THE ACFT WAS TRACKED ON BRITE RADAR OUT THE CONTROL ZONE. THE ACFT WAS CALLED IN AS OVERDUE AT 2354 GMT (APRX 1 HR AFTER DEPARTURE). WHEN THE ACFT WAS LOCATED THE NEXT MORNING, DIVERS FROM THE CG FOUND A FEMALE BODY IN THE ACFT. THE DROWNED PLT WAS FOUND ON A BEACH 11/11