What model Yanmar do you have? Mine is a 2003 4JH3E. The engine is installed under the galley. It has the raw water pump facing aft, below the alternator. A very poor design indeed. I have limited access to the pump through a small access panel, and I can just reach the pump. Other people seem to remove the alternator every time they need access to the pump. I don't . Impeller replacement became much easier since I replaced the cover with a Speed Seal. When the boat was built I ordered an after market engine mount bracket, as the original Yanmar design would not allow removal of the pump without removing one of the engine mount brackets first.
The manual says to replace the impeller every 600 hours. Don't believe it. I inspect the impeller once a year, and replace it at least every other year. It is cheep insurance. You don't want an impeller blade to break off and block