Use a 12 volt Sealed Lead Acid battery or SLA. Motorbike batteries have liquid electrolyte which will do a lot of damage inside a small enclosed space like a model boat if the battery tips enough some of the acid will come out.. There's no place for it to go except to stay inside the boat, which is bad. An SLA can be operated or charged in any position. In an SLA the electrolyte is contained in either a gel or in a fiberglass mat so it will not leak out unless the battery becomes physicall damaged.