500 50 hp Merc.

How do I replace the water pump in this engine ?

Drop lower unit,there are five nuts and one bolt under the trim tab.Remove the rubber ring on top of the housing and the three nuts and screw.When you pry off the housing the impeller will generally come with it so watch for the key and don't lose it,it holds the impeller in place.
It's a good idea to change the gaskets as well if they haven't been done in a while.Put a blob of grease or petroleum jelly on the key and stick it in place,slide the impeller on making sure the key stays in place,lubricate the housing,again petroleum jelly or washing liquid push it on working it in a clockwise direction then everything back as you took it off

Nice, thanks, I'll get to it today.