DONT DO THAT !!!!
you get 24V from two batteries by connecting the -ve of one battery to the +ve of the other then take the 24V from the +ve of one battery and the -ve of the other.
-ve feed----veXXXXX+ve---------veXXXXX+ve---+ve feed
Now if you take the +ve of both cell and the -ve of both cells and connect then together you end up shorting out both cells
If you boat suvives the fire from burning wire you will end up with flat batteries
If you want 12V either use a 24 to 12V converter or power the item from the battery which is connectrd to the boat -ve and supply the item from that battery +ve only.
ake sure you connect the item -ve to the battery -ve which is connected to the boat -ve. If you get the wrong battery you will end up with the item -ve at +12v in respect to the boat which can cause lots of problems.