We travelled to Marina Hemingway in the early 2000's on a Mirage N37 trawler. Many of you may recall the cover story about our trip in Passagemaker Magazine written by Peter Swanson. It is my opinion he is one of the real experts on travel to Cuba having made several trips since then.
Back when we went, it was perfectly legal to make this trip. It is my understanding you now need a license from the State Department, even when traveling by private yacht. To fly there from another country is not legal without this license (at least not when we went. Two women who had flown from Canada were arrested at the U.S. border upon their return. They immediately answered,"Cuba" when asked what other countries they had visited. Since they could easily book a flight from Canada, they thought it was legal.).
Track down Peter Swanson to get the latest and best advice. I don't recall the name of his website. I'll post if I recall.
We spent a week in Havana. A truly unequalled experience. The people were friendly. Sights unique. Great food.