I have replaced mine several times and it "Ain't" easy. First you have to take up that last section of floor that includes the access door that is too small to get into. Remove the caulk and the 6-8 screws and it comes up pretty easily. Then you can get in far enough to get to the impellor housing. There should be 4-SS wing nuts holding on the cover. Remove them, remove the cover, then use some needle nose pliers to yank out the old impellor. Be careful not to drop any of these because you won't get them back. (don't ask how I know) Make sure you note the direction that the impellor vanes are facing so you put the new one in the same way. Make sure you order a couple extra wing nuts along with your new impellor - you will need them because you will drop at least two of them into the generator housing trying to get them back on. Once you drop them they are gone. (agian - don't ask me how I know) There are a few hoses and wires that are directly in front of the impellor housing. If you put a tie wrap around the entire bundle and temporarily pull them out of the way life will be easier - not a slam dunk but easier. OK- now you have your new impellor and extra wing nuts. Get in the access hole as far as you can so you can look into the impellor housing to see the direction of the keyway for the impellor. This is real important because once you get the impellor in you will never line it up unless you are close to begin with. Now the tough part.
The new impellor is not very flexible and you will never be able to bend all the vanes in the same direction and get it in the housing, line up the keyway and shove it in with one hand. So here's what works for me. I take a tie wrap and place it around the impellor and pull it tight until the vanes are folded enough and in the proper direction to fit in the housing. I push in the new impellor as far as I can making sure that the keyway is lined up, then I cut the tie wrap. This may take some practice. But it does work. To make all this even more difficult the impellor needs to be coated with dishwasing liquid or mineral oil before you put it in. Now you are really having fun with that slippery little bastard!! Now the 2nd hardest part is getting the wing nuts back on. There are 4 of them and only one is easy. The tendency is to put the easy one on first - wrong. If you do that it will be in the way of all the others and you will just end up taking it off after an hour of fighting with the others. Start with any of the other ones and do the easy one last.
Hints - don't attempt any of this if you don't have at least 1 1/2 hours of uninterupted time. Don't drink any caffeine it it makes you jumpy. A beer or two will mellow you out just about right. Take your time and finish the 6 pack once you are done - you will need it.
Once you finish this job you will understand why most marine mechanics have the body of a 12 year old boy.
Good luck and let me know if you have any questions.