DURING THE FLT, THE PLT CONTACTED VERO BEACH TOWER & REQUESTED THE WX. HE WAS ADVISED THAT VERO BEACH & FT PIERCE, FL WERE VFR, BUT THERE WAS WX NORTH OF VERO BEACH, A CONVECTIVE SIGMET WAS IN EFFECT & THERE WERE THUNDERSTORMS IN THE AREA. THE PLT ACKNOWLEDGED THE TRANSMISSION & INDICATED HE WAS IN THE AREA OF THE VALKARIA ARPT. SHORTLY AFTER THAT, HE REPORTED THAT HE HAD OVERFLOWN THE VALKAPIA ARPT & WOULD PROCEED SOUTH TO VERO BEACH. ABOUT 4 MIN LATER, HE ADVISED THAT HE WAS NORTH OF VERO BEACH IN IFR WX & THAT HE WAS GOING TO REVERSE COURSE & GO BACK TO VALKARIA. THE PLT'S LAST RADIO TRANSMISSION WAS MADE AT 1820 EDT. THE NEXT MORNING, ACFT WRECKAGE WAS FOUND IN THAT SAME VICINITY WHERE THE PLANE HAD IMPACTED IN THE INTERCOASTAL WATERWAY. AN EXAM OF THE WRECKAGE REVEALED THAT IT HAD CRASHED IN A NEAR INVERTED, NOSE DOWN, LEFT WING LOW ATTITUDE. AN EXAM OF THE WRECKAGE REVEALED NO EVIDENCE OF A PREIMPACT MECHANICAL MALFUNCTION OR FAILURE. INTENSE THUNDERSTORMS, LIGHTNING & HEAVY SHOWERS WERE REPORTED IN THE AREA.