I've never been a cruise, and I know little about them, but I hope that something among my random thoughts about your post might be useful to you:
1. It might be worth considering Saga cruises. They restrict travel to over-50s (except that a travelling companion over-40 is permitted to join an over-50 passenger):
http://www.saga.co.uk/travel/cruises3/sagacrui singhome.asp
2. To find the best cabins on any ship, simply look for the most expensive ones! They will be the ones on the outside of the ship (so that you get a sea view) and on the upper levels (so that they're away from the engines and closer to the restaurants and other facilities).
3. The electrical supply on passenger ships is, to the best of my knowledge, always 110 volts AC. You should check the scooter's handbook to see whether it can be charged from such a supply.
4. While most cruise lines will try to be 'disability-friendly' you need to ask lots of questions (preferably getting answers in writing) about access onto and within the ship. Additionally you need to find out about any tours included with the cruise (e.g. Can the coach, from the port, accept a mobility scooter? Are there any access problems at the places to be visited?).
5. Some of the links here might be useful:
http://www.cruisecritic.co.uk/cruisestyles/are a.cfm?area=30
6. Some of the advice here might be relevant:
http://www.luxurycruiseholiday.co.uk/cruising- with-a-disability.htm
Chris