none of the water pumps are very hard to change. I did mine on a 100 evinrude last year in 30 minutes. get your self a manual for your engine if you do it your self you save a ton of time and money.
Here is a link to an article I wrote on water pump/impeller replacement on a 150 Merc. Hope it helps for your job. As stated, not too hard to do with basic shop tools and an extra set of hands.