re: boat battery help needed

1st check both new batteries in the 24V system. If you have a volt meter, disconnect the batteries from each other, then check for 12+ volts. If no volt meter connect each battery to your engine to see if they will crank just for diagnosis, If both are good you have an open in your circuit, The obvious is corroded cables. Not just the terminals but you can also have corrosion inside the cables. If the cables are good, check your switch wires and the relay between the switch and the battery cables. You can get an inexpensive Volt/Ohm meter at a parts house lie Auto Zone. Good Luck.