Presumably a tip to the person fueling the boat up at the marina. Customary to give a little tip.
I fill gas cans when going to the lake, but once I’m there the marina makes more sense. It takes me an hour to go get gas from the cottage and will cost me $ 10 in gas . So if the marina gas is is $2/ gal more I’ve already spend a dollar of that savings going to the gas station. so now the question is “ is that hour of lost time worth 10 dollars. To me the answer is no. ( assuming your buying 10 gallons of gas )