A common problem with batteries is deterioration due to not being stored correctly when not in use, causing it to loose its charge capability. in many cases this cannot be rectified once this has happened and you'll probably be in for a new battery.

Battery packs cause many problems in themselves. They are made up of a number of cells all wrapped up in heat shrink. This means that if just one cell starts to deteriorate it will bring the capability of the other cells down with it.

You can easily tell which cell is dying by feeling which cell is the hottest after it has been placed on load. They should all be equally as hot.

If the boat is not performing as well as usual try just checking all the connections. it's also a good idea that any heavy duty batteries do not go through a switch.