EMPLOYEE STATED HE WAS STANDING ON 636 SCOOP BLADE WHILE PREPARING A PANLINE KEEPER FOR INSTALATION. EE STEPPED BACKWARD OFF SCOOP BLADE (11"" FROM MINE FLOOR) ONTO A ROCK WHICH WAS 6 1/2"" UNDER WATER & FELL ONTO HIS BACK INTO THE WATER. WHEN HE GOT UP HE INFORMED MGT. THAT HE HURT LEFT ANKLE. EE WALKED (WITH HELP OF 2 MEN) TO TRANSPORTATION OUT. TALKED TO SURFACE E.M.T.