Salt water is better and they have come a long way with the newer powder coated (definately ask) paint finishes. My rusts all the time, it's called rusty metal primer, followed by flat black

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Salt water demands 24V minimum, but if you have the room for 3 batteries, my 36V never dies; even after 5 days in Maine camping and never bumping once. But on the ocean in wind and waves, the 36V comes in handy. Go longest shaft your boat will comfortably support, and on board charger.