ON DARK (MOONLESS) NGT, NONSCHED/DOMESTIC/PSGR FLT DEPD BLOCK ISLAND FOR WESTERLY, RI, WITH 7 PAX & 2 DOGS. IT CRASHED ALONG RTE IN BLOCK ISLAND SOUND, 3 TO 5 MI NW OF ISLAND (AN AREA LACKING VIS REF ON SFC). LTR, MAIN WRECKAGE WAS FND IN 130' OF WTR. ADNL WRECKAGE & BODIES OF PLT, 3 PAX & 2 DOGS WERE FND. ACFT WAS BADLY DMGD DRG IMPACT (L WING SEPD OTBD OF L ENG, EXTSV DMG TO FWD PART OF ACFT), BUT NO PREIMPACT PART FAILURE/MALFUNCTION WAS FND. COML/INST RATED PLT WAS PART TIME EMPLOYEE. HE WAS ON MEDICATION FOR LYME DISEASE, BUT NO PHYSIOLOGICAL PRBLM WAS FND THAT WOULD HAVE RESULTED IN ACDNT. AT APRX 1720 EST, THERE WAS A PIREP OF A BKN CLD LAYER AT 10,000'. ANOTHER PLT, WHO WAS FLYING AT ABT THE SAME TIME, RPRTD SCT CLDS ABV 10,000', OVR 10 MI VIS, WND WESTERLY AT 15 TO 25 MPH & LGT TO MOD TURBC. SIGMET (GOLF 4) WARNED OF MOD OCNL SEV TURBC BLO 6000' & POTENTIAL OF LOW LVL WND SHEAR. ACDNT OCCURRED ALONG BIRD MIGRATORY RTE. THERE HAD BEEN NO RECENT FAA INSPN OF BASE, RAMP, COCKPIT (EN RTE CHECK), GND/FLT TRNG PROGRAM, CREW/DISPATCH/TRIP RECORD.