Short version:
Drain oil from gearcase.
Disconnect clutch cable from bottom of clutch lever (below gas tank)
Remove the curved tube that the cable is coming out of.
Remove 4 bolts holding gearcase to midsection.
Notice the 1/4" plate between gearcase and midsection. Remove gearcase from below the plate.
Now pull the plate assembly from midsection. Water pump and clutch & cable will come with it.
Takes a special impeller (NLA). you can make one by reaming the center hole in a 3hp / 5.5hp / 7.5hp impeller