DRG ARR, TRACON GAVE THE PLT THE ARPT WX & PROVIDED VECTORS FOR AN ILS RWY 32 APCH. THE PLT INQUIRED ABOUT A CELL THAT HE UNDERSTOOD WAS OVER THE ARPT & WAS ADVISED TO STANDBY WHILE THE CONTROLLER (CTLR) CHECKED ON IT. THE CTLR CONTACTED TWR PSNL WHO ADZD THE VIS WAS 1 MI IN HVY RAIN, BUT THE PLT WAS GIVEN A FREQ CHG BFR THE INFO WAS PASSED ON TO HIM. HOW-EVER, THE FINAL CTLR ADZD HIM OF THE CELL WHICH WAS OVR THE NDB. THE NDB WAS 1.6 MI FM RWY 32, ALONG THE ILS COURSE. THEPLT ELECTED TO CONT THE APCH & WAS CLRD TO TWR FREQ. AS THE ACFT APCHD THE NDB, RADAR SHOWED IT WAS OFF COURSE. THE TWR CTLR TRIED TO GIVE THE PLT CLIMB-OUT INSTRNS, BUT THE PLT DECLINED THEM & SAID HE WAS HAVING TROUBLE CTLG THE ACFT. THE PLT'S LAST TRANSMISSION WAS TO ACKNOWLEDGE HE WOULD TURN 180 DEG TO ABANDON THE APCH. WITNESSES SAW THE ACFT COME OUT OF CLOUDS IN A STEEP DIVE & CRASH ABOUT 1 MI FM THE ARPT IN 10' TO 15' OF WATER BFR RECOVERING. NO PREIMPACT MECHANICAL PROBLEMS WERE FND. A WX STUDY SHOWED CLOUD BASES AT 700' & WX ECHOS OF AT LEAST MOD INTENSITY (VIP LVL 2).