THE STUDENT WAS A COMMERCIAL PLT WITH RATINGS IN SINGLE & MULTI-ENG LAND & SINGLE ENG SEA ACFT. HE WAS ON HIS 1ST FLT IN THIS MAKE & MODEL OF ACFT TO RECEVE INSTRUCTION FOR A MULTI-ENG SEA RATING. AFTER INSTRUCTING WATER LANDINGS WITH FLAPS, THE INSTRUCTOR TOLD THE STUDENT TO MAKE A NO-FLAP LANDING ON THE WATER. ACCORDING TO THE STUDENT, THE LANDING APCH LOOKED NORMAL. WHEN HE RETARDED THE THROTTLES TO IDLE IN THE FLARE, HIS INSTRUCTOR SAID SOMETHING IN A LOUD VOICE. THEREAFTER, HE FOUND HIMSELF UNDER WATER. AFTER DISCONNECTING HS SEAT BELT, HE CAME TO THE SURFACE & WAS EVENTUALLY PICKED UP BY PERSONNEL IN A BOAT. THE STUDENT THOUGHT A DOOR MIGHT HAVE COME OPEN, BUT HE DID NOT KNOW. MOST OF THE WRECKAGE WAS RECOVERED. THE COCKPIT DOORS WERE NOT RECOVERED, BUT THERE WAS EVIDENCE THAT THEY HAD SEPARATED DURING IMPACT. MAJOR IMPACT DAMAGE TO THE ACFT WAS IN THE NOSE & COCKPIT AREA. NO PREIMPACT FAILURE WAS FOUND. THE 1250 EDT WIND AT LAKELAND, FL (7 MI WEST) WAS CALM. NO SHOULDER HARNESS INSTALLED, CFI FATALLY INJURED.