In the "old" days, there were greater differences in fuel-oils from "truck stops" for a number of reasons (e.g., in colder climates the mixture was "thinned" for winter). However, today with EPA regs, there is little difference other that much more sales taxes and a more sheltered market for marine fuel locations.

I think the biggest difference is DON'T go to a place where they don't pump a lot of fuel -- whether it be by water or wheels.

For a clearer explanation of the differences between fuels in Marine (as in pleasure vs. freighter-commercial fuels):

http://www.epa.gov/otaq/regs/nonroad/marine/ci/fr/dfuelrpt.pdf