I don't think it is so much that they have a captive audience as it is the lack of volume.
It's not like most marinas have 30-40 customers an hour rushing in to purchase fuel, drinks, snacks like your average AM-PM that is making 20 cents on a gallon of gas and $1.25 profit on that "Big Gulp". Some of these marinas are probably lucky to see a couple of fishermen a day during the week. This does not even tank into account the environmental issues that a marina fueling dock is faced with.
It's not like most marinas have 30-40 customers an hour rushing in to purchase fuel, drinks, snacks like your average AM-PM that is making 20 cents on a gallon of gas and $1.25 profit on that "Big Gulp". Some of these marinas are probably lucky to see a couple of fishermen a day during the week. This does not even tank into account the environmental issues that a marina fueling dock is faced with.