To be clear, here is current language from the USCG website (http://www.uscg.mil/hq/cg5/cg531/CubaTravel.asp):

"Failure to comply with the Coast Guard, Commerce, Treasury, or other Federal government regulations regarding travel to Cuba will subject violators to federal criminal prosecution, as well as possible administrative proceedings by the Department of Commerce and Department of Treasury. Penalties for violations of these Federal statutes and regulations can result in fines, imprisonment, vessel seizure and forfeiture, and denial of future export privileges.
If you do not have all of the appropriate permits and licenses required by the Department of Commerce and the Department of Treasury, and you make a voyage into Cuban territorial waters, you are subjecting yourself to any and all of these sanctions. You are hereby advised that, in compliance with direction from the President, the U.S. Coast Guard will be stringently monitoring maritime traffic to and from Cuba in order to ensure that vessels subject to U.S. jurisdiction have complied with all applicable licensing requirements, laws, and regulations, and will take aggressive enforcement actions if those conditions are not met."

There are strict regulations, and serious fines and penalties that are still in force in regard to trading with Cuba. The charter broker is not signing the charter contract - it is the "Owner" who is entering into trade with Cuba.  As such, it is the owner's responsibility to verify that all persons (crew and guests) are qualified to visit Cuba under the U.S. Visa requirements, and it is the owner's responsibility to see that the charterers adhere to the U.S. visa requirements in Cuba. And it is the Owner's responsibility to retain records in case of a Treasury audit for 5-years after the charter. There are also regulations in Cuba that must be adhered to, and many of these are not "published". However, when you break these rules, you will learn about them, and the consequences. And if a charter is deemed to be 'operating illegally, under either U.S. Laws, or Cuban Laws, the vessel's insurance may not be valid, in event of a claim.

We recommend that owners, and their counsel, contact the US Treasury Department directly to find out more: 

Department of the Treasury
1500 Pennsylvania Avenue, NW 
Washington, D.C. 20220
Tel: (202) 622-2000