If their contribution is VOLUNTARY and actually limited to a portion of your expenses while they were aboard, it is likely to be considered personal, not business. There would seem to be enough gray area on the laws so that a major question is: Did you know this person beforehand? Or did you solicit them with the understanding they'd be paying you?
The law used to be simple, any "consideration" including buying the beer for your uncle when you got back to the dock made the whole trip commercial. Then the USCG lightened things up--with the intention of allowing FRIENDS to buy lunch or share the gas with their friends.
I'd still say if you are advertising and making deals with strangers--that's commercial. Try it, have a problem that goes to court, and see how the courts rule. It is only their opinion that counts--but that's how the laws are intended.