Didn't know that about the enforcement across the waters interesting.
To clarify: Federal Law in the US says that every child 12 and under on a vessel underway must wear a properly fitted USCG approved flotation device, unless said child is below decks in a berth or in a cabin. There are exclusions for passenger ferries and such, but for recreational, they must have them on.
As of yet, there is no requirements for adults or teens to wear them (though local or state laws may overrule that); but the vessel must have at least one Type 1 (offshore, 22lb buoyancy), 2 (nearshore, 15.5lb), or 3 (buoyancy aid, 15.5lb) PFD on board for each individual who is not wearing a Type 5 (Special Use) PFD.
Local and State laws may restrict usage (mandating usage to higher degree then the feds require), but they cannot relax the Federal requirements.