12 volt to 24 volt conversion
It is the total amperage of your battery system that determines how much power you have available. Increasing the voltage is only advantagous if you are powering heavy current demanding loads;ie, thrusters, large windlasses, etc. Then, it only saves you the added wire size needed to carry the additional current. There is no free power. And then, you will have to charge the batteries. Without a bunch of diodes to isolate each set of 12vdc batteries, your charging system will have to be upgraded as well. My suggestion would be to put in the additional batteries and go.