THE PLT RECEIVED A WX BRIEFING AT 1037 EST BEFORE DEPARTING ON AN IFR X-COUNTRY. AT 1051, A SEVERE THUNDERSTORM WARNING (#537) WAS ISSUED BY THE NATIONAL WX SVC. WHEN THE PLT REQUESTED AN IFR CLEARANCE AT 1107, HE WAS ADVISED OF THE FRONTAL ACTIVITY. ABOUT 3 MIN LATER, HE WAS BRIEFED ON THE WX WARNING, WHICH HE ACKNOWLEDGED. AT ABOUT 1138, THE PLT WAS ADVISED OF WX IN HIS VICINITY & ALONG HIS PLANNED ROUTE WHICH INCLUDED MODERATE TO SEVERE TURBULENCE & MODERATE TO HEAVY RAIN. BASED ON THIS INFO, THE PLT ELECTED TO DIVERT TO SAVANNAH, GA. AT 1139:54, HE REPORTED HIS WX CONDITIONS WERE ROUGH WITH RAIN. AT 1141:11, THE CONTROLLER NOTED THAT HIS TRANSPONDER REPLY SHOWED 2600 FT INSTEAD OF THE ASSIGNED ALT OF 7000 FT. AT ABOUT THAT TIME, RADIO & RADAR CONTACT WERE LOST. THE ACFT WAS PRESUMED TO HAVE CRASHED AT SEA & BOTH PLTS WERE PRESUMED TO HAVE BEEN FATALLY INJURED. A SEARCH OF THE SUSPECTED AREA WAS MADE BY THE COAST GUARD & DEBRIS SUSPECTED FROM THE ACFT WAS FOUND; HOWEVER, THE MAIN WRECKAGE WAS NOT RECOVERED.