THE STUDENT & FLT INSTRUCTOR (CFI) TOOK OFF ON A TRAINING FLT IN A ROBINSON R22 MARINER (FLOAT EQUIPPED) HELICOPTER. DRG THE RETURN FLT FM A PRACTICE AREA, THE HELICOPTER CRASHED. BFR THE ACDNT, 2 WITNESSES HEARD THE HELICOPTER, BUT DID NOT SEE IT UNTIL AFTER THEY HEARD AN UNUSUAL NOISE. ONE WITNESS SAID HE 'HEARD A LOUD PING, LIKE METAL AGAINST METAL.' AT ABOUT THAT TIME, BOTH WITNESSES NOTED AN INTERUPTION IN ENG PWR & SAW PIECES COMING OFF THE HELICOPTER AS IT FELL TO THE GND. IT CRASHED IN AN INVERTED ATTITUDE & BURNED. AN EXAM OF THE WRECKAGE REVEALED THE MAIN ROTOR BLADES HAD STRUCK THE TAIL BOOM & THE ENTIRE TAIL ASSEMBLY (INCLUDING STABILIZERS, TAIL ROTOR GEAR BOX & TAIL ROTOR) HAD SEPD. THE TAIL ASSEMBLY FELL TO THE GND APRX 250' FM THE MAIN WRECKAGE. NO PRE-EXISTING MECH PROBLEMS OR SEVERE MAST BUMPING WERE EVIDENT. THE CFI HAD 451 HRS OF FLT TIME, ALL IN R22 HELICOPTERS. IN THE PREV 5 DAYS, HE HAD FLOWN 2 HRS IN FLOAT EQUIPPED (MARINER) R22'S & HAD MADE WATER LANDINGS, BUT NO INSTRUCTION IN THE MARINER OR ITS DIFFERENCES WAS VERIFIED.