For most boats under about ten metres the answer is 12 volts, because that is what the equipment on the market assumes. As boats get bigger, and as loads like thrusters, windlasses and inverters get larger, the cable required to carry the current at 12 volts starts to become the deciding factor.
Doubling the voltage halves the current for the same power, which means smaller cable, less voltage drop and easier installation.
Long cable runs, high current loads and larger inverter systems all favour 24 volts. So does a boat that already has a 24 volt engine start system, since staying with one voltage is simpler than not.
Equipment choice is wider and usually cheaper at 12 volts, from instruments and lighting to pumps and fridges. On a modest boat with short runs, the theoretical advantages of 24 volts rarely repay the extra complication.
Mixed systems are common and perfectly workable, usually with a DC to DC converter feeding a 12 volt sub system from a 24 volt main bank. What matters is that it is designed rather than improvised: separate distribution, correct fusing on both sides, clear labelling and no accidental cross connection.
The commonest problem we find is a 12 volt item quietly tapped across half of a 24 volt bank. That unbalances the bank and shortens its life, and it should never be done.
Converting an existing boat is a big job, because every appliance, light and pump either changes or needs a converter. It is worth doing during a full rewire, and rarely worth doing on its own.
No. It unbalances charging across the bank and damages the batteries. Use a DC to DC converter instead.
It reduces losses in cable for the same power, which matters on long runs and high loads. On a small boat with short runs the difference is minor.
Yes, with a properly designed converter and separate, correctly protected distribution for each voltage.
We design the DC system around the boat and the loads, then quote the installation in writing.