1998 Mercury 9.9/15HP 4-stroke outboards factory service manual, Page 323Get this manual
BIG FOOT
Water Pump
1Slide new face plate gasket and face plate over driveshaft and position them on gear housing2Install nylon washer over driveshaft and set it flat against face plate3Place impeller drive key on flat of driveshaft4Slide new impeller over driveshaftAlign impeller keyway with drive key, then push impeller over drive key and against face plateIf reusing impeller (not recommended) note orientation of vane set5Install nylon washer and set it flat against impeller6Install new O-ring in water pump coverInstall cover assembly over driveshaft and down against impeller