This thread is rapidly turning into a political/historical debate. I would suggest people who have not been to Cuba by boat/do not have relevant information for w1651 about the procedures, precautions, and preparations to make for his potential voyage. I don't think the rightness and justification for the embargo and government positions should be addressed here, simply how, what, and when the author could go about this trip if he decides to. It comes down to a personal choice, and those who have already posted have made clear the benefits and shortcomings. Let's stop the politics now and offer relevant responses to the original question.
I personally know many cruisers who have gone to Cuba by boat and had a wonderful time, with no problems from the US government later on. I would say plan on taking additional time and don't cruise to only Cuba if you are able. There are many other places worth seeing, and it may be safer/easier to return from one of these places. Also consider taking a longer sail to re-enter the US in a more northern port such as Pensacola or New Orleans if you want to allay concerns raised about the Customs agents in ports such as Marathon, it is only a few days further to sail up the gulf and you can have an enjoyable sail back to St. Pete along the US coast.
Where did your friends enter Cuba? Did they sail around the island or stay at one port and travel from there?