Let's quote from the Coast Guard, since they're the ones who interpret the requirements, advise on their creation, and arrest violators. They're really very clear on the subject.
The ultimate source, for US boats and boats trading in US waters would probably be US Code Title 46 (Subpart C is mentioned in the USCG doc below). The sobriety requirement for skipper and crew would eliminate some sailors I know from legally carrying passengers for hire, even if they did have a license!
Passenger means an individual carried on the vessel except:
a. The owner or an individual representative of the owner or, in the case of a
vessel under charter, an individual charterer or individual representative of the
b. The master; or
c. A member of the crew engaged in the business of the vessel who has not
contributed consideration for carriage and who is paid for on-board services
(46 U.S.C. 2101(21) (A)).
Passenger for hire means a passenger for whom consideration is contributed as a
condition of carriage on the vessel, whether directly or indirectly flowing to the owner,
charterer, operator, agent or any other person having an interest in the vessel (46
Consideration means an economic benefit, inducement, right, or profit including
monetary payment going to an individual, person, or entity. It does not include a
voluntary sharing of the actual expenses of the voyage, by monetary contribution or
donation of fuel food, beverage, or other supplies (46 U.S.C. 2101(5a)).
U. S. Coast Guard Requirements for Uninspected Passenger Vessels of Less Than 100 G.T. and Carrying Six or Less Passengers
(and yes, I would have nitpicked it as "Six or Fewer Passengers" but does anyone see an angry grammarian around)
Licensing (46 CFR 15.605, 46 CFR 26.20-1 and 46 CFR 15.401)
Each self-propelled, uninspected vessel carrying not more than six passengers as
defined by 46 U.S.C. 2101(42) (B), must be under the direction and control of an
individual licensed by the Coast Guard
While on board an UPV, a crewmember (including a licensed individual), pilot or a
watchstander who is not a regular member of the crew:
a. Shall not perform or attempt to perform any scheduled duties within four hours of
consuming any alcohol;
b. Shall not be intoxicated at any time;
c. Shall not consume any intoxicant while on watch or on duty; and
d. May consume a legal non-prescription or prescription drug provided the drug
does not cause the individual to be intoxicated.
A hazardous condition can be Operating without a license, operating beyond the scope
of the license, or operating with insufficient crew for the intended voyage. Boarding
officers shall review the licenses of all licensed personnel required. A licensed operator
may not turn the watch over to an unlicensed person while sleeping or resting off the
bridge. The practice of permitting passengers on uninspected passenger vessels to
"take the helm" should be discouraged, however, it is not prohibited by regulation. The
licensed operator is responsible for the safe navigation/operation of the vessel at all times.