Peter,
Your amp usage will be dependent on a number of factors so running your figures is just a starting point. Get the biggest batteries your boat and wallet will handle.
You may want to check out the Electric boats group at:
electricboats : Electric Boats. Almost any question possible has been answered there.
Also take a peek at
http://www.electricyacht.com/ . I got my system from them and got great support and service at a good price.