Some 24v systems actually employ a series of linked 12v or 8v batteries.
Charging 24 volt battery system.
The 24 volt charger is connected to the negative on battery 1 and positive on battery 2 inputting 24 volts to the series battery bank.
Cars trucks rvs and motorhomes run dual 12 volt batteries for various reasons.
Depending on how you wire a two battery 12 volt system the result can be a 12 volt system or a 24 volt 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.