BEFORE THE FLT, THE PLT STATED HIS INTENTIONS WERE TO PRACTICE NGT LNDGS. HE TOOK OFF AT 2010 CDT WITH AN ESTD 1.5 HRS OF FUEL ON BOARD. WHEN THE ACFT DID NOT RETURN, A SEARCH WAS INITIATED. SUBSEQUENTLY, IT WAS FND BY FISHERMEN, SUBMERGEDIN A LAKE APPROXIMATELY 15 MI SE OF THE ARPT. AFTER IT WAS RECOVERED FROM THE LAKE, AN EXAM REVEALED CRUSHING DAMAGE ON THE NOSE OF THE ACFT & THE LEADING EDGES OF THE WINGS. ALSO, THE NOSE GEAR SEPD, THE MAIN GEAR COLLAPSED TO THE REAR & THE AFT FUSELAGE WAS BENT DOWNWARD JUST AFT OF THE WINGS & MAIN GEAR. NO PREIMPACT MECHANICAL PROBLEM WAS FOUND. A WITNESS AT A BOAT DOCK SAID HE SAW AN ACFT (MATCHING THE DESCRIPTION OF N25991) FLYING OVER THE LAKE AT ABOUT 2030 CDT. HE SAID THE ACFT WAS FLYING EXTREMELY LOW, THEN CLIMBED OVER TREES, TURNED TOWARD THE NE & BEGAN CLIMBING. HE DID NOT SEE THE ACFT AFTER THAT & ASSUMED IT WAS RTRNG TO THE ARPT. A PATHOLOGICAL EXAM OF THE PLT REVEALED EVIDENCE OF RECENT &OLD MYOCARDIAL INFARCTION. THE CORONER BELIEVED THE CAUSE OF DEATH WAS RELATED TO HIS CARDIOVASCULAR CONDITION.