Having a 24 VDC electrical system aboard a boat has it benefits, such as smaller wire size which helps lessen effect of voltage drop. Unfortunately not all loads come in 24 VDC and you will inevitably run into the need to run some loads on 12V. You will need to power these 12 VDC loads via either a dedicated 12 VDC battery or use 24 to 12 VDC converter. In most cases, the simplest option is a converter. It can be sized according to its load, compact, and avoids the need for another battery. It is essential that the converter is sized to handle the load and with an adequate safety margin.
The rule of thumb is that the converter should be at least 125% of the rated load of the components it will power and ideally 150% of the rated load should be the target. If there are any doubts about the load that is expected then a simple solution is an energy audit that can be performed by a clamp-on multimeter so that all loads connected to the batteries are accounted for (except of course those that are connected directly to the battery). Or a professional that can calculate the needs of your specific electronic, size and install the converter.