Ian, using a lead acid battery out of a motorcycle may not be a good idea if its wet acid. Unlike the sealed lead acid type, a wet acid battery can and will spill, and if it does, it will very quickly eat a hole in the boat.
battery acid is sulphuric acid, you don't want that in your boat, also the fumes can be corrosive as well.
BEST USE A GEL CELL and leave the other battery for your motorcycle