It is not necessary to cut an access holt for the water pump impeller. It has been mentioned that the Alternator has to be removed, it does not. I padded the top of the engine with a couple of large folded towels so I could lay over the engine, did not remove the altinator or anything except the pump cover, extract the old impeller, install the new impeller ( the gap between the pump housing and the starter end is just enough to work the new impeller into place) replace the o ring and cover, remove the back end of the heat exchanger to check for fragments of the old impeller and replace end cap, you are done.
The bottom inside cover screw is difficult to get started but not that bad. I read about access holes and would never cut my boat like that (there would be a raw hole on the engine side of the wall or a need to place a cover on both the head and engine side of the wall.)
Not sure if they are still available, there was a cover with two slotted holes that allowed you to loosen the two hardest to get to bolts and remove the two easy ones to get the cover off.
Lubricate the new impeller and place a hose clamp around It to compress it to allow easy insertion into the housing. I vote no access hole.