Just like Grant said, put the alternator input to the A terminal, and the two battery positives to terminals 1&2. Use big wire. Run the negative wires to the same ground point on the block. Better yet, get someone who understands high current DC boat wiring to help or do it for you. You don't want to set your boat on fire. I'm not being a smartass, the consequences are huge and potentially life threatening if you don't know what you are doing. Spend 200$ to get it done properly.