I am surprised no one has mentioned the possible
legal issues. Are you an American citizen? Is the
boat a US documented vessel? If either is true, you should do a lot more
research before deciding you can leave
Miami and just pull into Havana to refuel. US citizens are prohibited from visiting
Cuba (there are exceptions none of which will apply to you).
Purchasing fuel will be an additional violation of the "Trading With the Enemy Act." While there is a probability you may not be stopped or bothered, the downside if you are is very expensive and may cost your
delivery customer his
boat. Leaving
Miami and heading straight to Havana at 24 knots is a very good way to attract a lot of unwanted attention. You might want to chart an alternative course.