THE HELICOPTER WAS ON A LCL PHOTOGRAPHY (SIGHT-SEEING) FLT. WHILE HOVERING AT APRX 200' ABV THE POTOMAC RIVER, THE ENG LOST PWR. THE PLT INITIATED AN AUTOROTATION & DEPLOYED THE EMERG FLOATS. SUBSEQUENTLY, THE HELICOPTER IMPACTED IN THE RIVER & ROLLED OVR, BUT WAS SUPPORTED (UPSIDE DOWN) IN THE WATER BY THE FLOATS. AFTER RECOVERY, NO PREIMPACT PART FAIL- URE OR MALFUNCTION OF THE AIRFRAME WAS FND. HOWEVER, A TEARDOWN OF THE ENG REVEALED THE SPUR ADAPTER GEARSHAFT (SAG), PN 6890482, HAD FAILED FROM WEAR & SUBSEQUENT FATIGUE. THE FAILURE OCCURRED AT THE FWD, SPLINED END OF THE SAG. DEBRIS (82% SILICON) WAS FND INSIDE THE OIL DELIVERY TUBE. THE OIL JETS FOR THE #2 & #3 BRGS WERE OBSTRUCTED, BUT THE JET FOR SAG WAS NOT OBSTRUCTED. THE PRECISE NATURE OF THE CONDS & CIRCUMSTANCES THAT LED TO EXCESSIVE WEAR WAS NOT DETERMINED. THE HEIGHT/VELOCITY DIAGRAM IN THE FLT MANUAL SHOWED HOVERING FLT BTN 10 & 400 FT SHOULD BE AVOIDED (TO ALLOW FOR SUCCESSFUL AUTOROTATIVE LNDG IN THE EVENT OF A PWR LOSS).