I want to do some maintenance on my Merc 2s. How difficult is it to change a water pump impeller, and also the overheat alarm sensor unit? a) Pretty easy if you can turn a wrench and follow directions. (about as difficult as changing the trans filter and fluid on a car). b) Moderate, if your mechanically inclined and follow the outboard manual directions carefully should not be bad. (about as complex as removing and cleaning carbs). C) Leave it to a pro. Easy to strip corroded bolts, need diagnostic equip and experience. Likely that a diy'er will waste a sat. and still have to take it in.