How tough is it to change water pump impeller on 15hp johnson

There is no special tricks.Unbolt the gearshift linkage that has access through the side of the leg.Usually a small plate.Unbolt the lower g/box.4 bolts I think.The box should then drop.If not,use a rubber hammer and be patient.Put the lower skeg of the g/box in a vice to make things easier.NOW.When you lift the housing of the w/pump take a note of which way the impellar pump lobes are curled.DON'T put the new impellar in the wrong way.It's probably worth putting in a new seal whilst it's apart.You spin the housing or imp. as you put it together.Use a small bit of grease for the initiall dry start-up.Put some very good grease on the top splines on the driveshaft.It goes straight into the crankshaft.If it corrodes,nothing comes apart.As you put the g/box back on,spin the prop so the splines go back in.There is no special alignment.Gearchange linkage and you go fishing.(take the prop off and regrease the splines on the shaft of it)Hope this helps