Cars trucks rvs and motorhomes run dual 12 volt batteries for various reasons.
Charging 24 volt battery system.
If your boat has a 24 volt battery system it probably has two 12 volt batteries that are wired together so that the negative post of the first battery connects to the positive post of the second battery.
The 24v systems that employ 12v batteries can be recharged with a 12v alternator by simply being plugged in like any other 12v battery.
If your charger is small enough to be carried aboard your boat you can leave these cables in place since the charging procedure requires it.
While most vehicles use a 12 volt 12v electrical system many buses and some boats utilize a 24 volt system.