No, the one next to the battery on the positive side is what is required. Not sure why he wired it the way he did and I'd change the main switch to the positive side also.
Since the boat is fiberglass you could control the circuit on the ground side but I sure wouldn't and can visualize some situations that might occur that might not have a happy ending. If the positive side can find another source to a ground then the switch on the negative side won't turn the battery off. On the positive side you for sure are going to turn the power off,
Sum
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