DRG TKOF CLB, A LOUD NOISE OCCURRED AS 1 OF 5 MAIN ROTORS (MRB'S) & THE TAIL BOOM SEPD FM THE HELICOPTER (HEL). THE HEL THEN CRASHED & HIT A SUBMERGED REEF APRX 200' FM THE HELIPORT. AN EXAM OF THE FAILED MRB SHOWED ITS RETENTION STRAP ASSY(RSA), PN 369D21210-501, FAILED FM FATIGUE IN AN AREA OF CORROSION; 11 OF THE MRB'S 16 STRAP LAMINATES HAD PREEXISTING CRACKS. CORROSION WAS FND ON 78 OF 80 LAMINATES. THE MAINT HNDBK REQD THE RSA'S BE INSPECTED AT 25, 100 & 600 HR INTER- VALS. SVC INFO NOTICE (SIN) DN-77.1 RDMDD A 300 HR INSPN IF ANY CRACK OR LAMINATE FAILURE WAS FND DRG A 100 HR INSPN. AD'S 77-15-09R1 & 81-10-08 REQD INSPN W/I (BUT NOT OUTBRD OF) THE PITCH HOUSING. DRG 100 HR/AD INSPNS AFTER 1768 & 1839 HRS OF OPN, UP TO 3 CRACKS WERE FND IN LAMINATE LEGS, BUT THEIR LCTNS WERE NOT DOCUMENTED FOR LTR INSPN. HOWEVER, DRG 2 SUBSEQUENT 100 HR INSPNS BY ANOTHER MECH, NO CRACKED LAMINATES WERE NOTED. THE HEL HAD 2135 HRS OF OPN WHEN THE ACDNT OCCURRED. SVC INFO LTR DL-54 RCMDD A TRI-FLOW WASH PROC AFTER THE LAST FLT OF EACH DAY WHEN OPNG IN A MARINE ENVIRONMENT