I have some good news, and some bad news. When I first called Carver, as my 2002 356 water heater had an internal leak (still not sure if this was an electrolisis issue, manufacturer failure, or something else) they said - hmmmm - never had to change one before, let us know how you do it! ouch. Now that I have a 506, and have had to do major work, including a stbd engine repower, they said the same thing. Let us know how to do it. Pretty lame.
Anyhow - Its a tough job. I'm 6'2, 205 and ended up scrunched up like a circus migit along side the thing in that little space. I took one of those tiny screw drivers that has philips on one end, and straight on the other, ground it down until I could turn it, and then one screw at a time, a quarter turn at a time, removed the dozen or so screws on top of the thing. I had a friend grab me by my ankles and pull me out of there so we could drink a few beers.
Basically, getting it out is not so much the concern, as the thing is already damaged. Once the top was free ( and electric removed, water removed, and bottom connections free) we just beat the hell out of it, until we got it out.
Mine was an 11 gallon Seaward, if memory serves, and they were great to deal with. I put the same unit in, which required a tiny bit of cutting to the fiber glass, but not too much.
I also had to replace the aft head solenoid, as well as DC house bank charger, and can advise you on both topics as well. My advice - now that I realize that Carver doesnt produce a 'shop manual' for any of this stuff - is to use your digital camera, and create your own guide. Take pics to show exactly how things are plumbed, wired, etc - and use a lap top / digital camera combo as a guide when re-installing.
Good luck - Ken