THE PLT & PSGR DEPD ON A LOCAL FLT AT NIGHT IN CLEAR WX CONDS. AFTER ABOUT 15 MIN OF FLT, THE ACFT CRASHED IN LAKE MICHIGAN, ABOUT 1 MI FROM THE SHORELINE. A WITNESS SAID HE SAW THE LIGHTS OF AN ACFT FLYING AT LOW ALT IN THAT AREA, WHICH HAD DISAPPEARED FROM HIS VIEW. ABOUT 2 DAYS LATER, WRECKAGE OF THE ACFT WAS FOUND IN ABOUT 50' OF WATER. THE PSGR WAS STILL STRAPPED IN THE ACFT. THE PLT'S BODY WAS FOUND ON 8/18/90 AFTER IT HAD WASHED ONTO THE SHORE. NO PREIMPACT PART FAILURE OR MALFUNCTION OF THE ACFT WAS FOUND. THERE WAS EVIDENCE THAT BOTH OCCUPANTS HAD DIED FROM DROWNING. TOXICOLOGY CHECKS SHOWED THE PLT'S BLOOD HAD A CARBOXYHEMOGLOBIN (CARBON MONOXIDE) SATURATION OF 21%, WHILE THE PSGR'S BLOOD HAD ONLY A 7% SATURATION LEVEL. (SMOKERS CAN BE EXPECTED TO HAVE A SATURATION LEVEL OF 3% TO 10%.) NO MALFUNCTION OR FAILURE OF THE ACFT WAS FOUND THAT WOULD HAVE RESULTED IN THE ELEVATED LEVEL(S) OF CARBON MONOXIDE.