I used to work at a marina. The reason the gas there is more expensive (aside from the monopoly theory) is because marinas don't sell as much gas, so they can't buy it in quantities large enough to receive the lower prices for buying in "bulk" (consider the winter months where they wouldn't sell near as much gas). That and IIRC, regulations with selling gas near open water are tighter.
ALSO, a lot of boats and PWC used to use mix gas, and a good percentage of those boats are still on the water (boats typically have a longer usable life span than cars). Mix gas is special order. Of course the new ones are now going to oil injection or 4-stroke engines.
Lastly, a good percentage of boats have high-compression engines, and cant use low-grade. So if they only have one storage tank, why wouldn't they choose to supply premium?
Anyway...
If it is illegal to full up at the gas station, we never got busted. We do it all the time.