Sure. I give you three reasons. You can determine the legitimacy of each, because I am not in the fuel supply business nor a mouthpiece for the industry.
1. Seasonal use. Marine fuel dealers have to make their annual profit in a five to six month season in your neck of the woods versus a twelve month season for highway suppliers. I don't know what their excuse is in the South.
2. Additives. You may be getting fuel pre-conditioned for marine use, such as the ValvTect blends, which do not need the user to purchase separate additional fuel additives.
3. To stay in business in NJ the dealer has to offset the nearly confiscatory tax burden of property taxes where he could, and maybe should, sell the place to developers who will turn your fuel stop into waterfront condos.
In my area diesel is $4.04/9 at the dock. Non-highway use diesel may be had for $3.55/9 at some gas stations which are set up to pump it that way. If you wish, you may fill and carry yellow 5 gal. cans to your boat.
Because I am an incredible cheapskate and because it is good exercise to carry the full cans, I sometimes do that. The downside is that it doesn't make much of a dent in a nearly empty 300 gallon tank to fill it 15 or 20 gallons at a time. When done for occasional fun to keep topped-off its not really a bad idea, but I'm not pushing it as your solution to high fuel costs.
Some prima donnas wouldn't carry a gallon to their boat, but that is entirely their decision.