Depending on how you wire a two battery 12 volt system the result can be a 12 volt system or a 24 volt system.
Charging 24 volt battery system.
The 24v systems that employ 12v batteries can be recharged with a 12v alternator by simply being plugged in like any other 12v battery.
Some 24v systems actually employ a series of linked 12v or 8v batteries.
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.
While most vehicles use a 12 volt 12v electrical system many buses and some boats utilize a 24 volt system.