On our boat that locker holds all our water bottles, and if there is still room, some beer cans. When we go on a month long cruise my wife wants to ensure we have three water bottles each per day--that's alot of water. We use our water tanks for dishes hand washing, etc. So that space can't be used for extra batteries on our boat. The two house batteries are in the battery compartment under the quarter berth, with an extra emergency start battery on a shelf in the port locker.
Frank