If your boating/fishing the salt then your in Coast Guard jurisdiction and they go by the CFR's (code of federal regulations) which basically says, if the boat owner accepts any kind of gift or money for his services he is a boat for hire/charter.
That being said, the marine safety officer that tested me for my license said he didn't personally know of anyone who had ever been cited for accepting "minimal" amounts of fuel money, I don't think he said specifically what "minimal" meant.
Personally, I wouldn't sweat it.