Well, yes. If you are a US citizen, you can't enter Cuba without US specific and personal permission. That isn't easy to get. There can be some permission granted for cultural, religious, educational or humanitarian reasons, but those definitions are much narrower than they sound as applied. We went a couple of years ago and it took us a year to get both Cuban and US permission. There are also expenditure limits per day on the part of the US and travel and lodging restrictions in Cuba. Further, there is an onerous, and required, exchange rate for Dollars to Convertible Pesos. US citizens do go in illegally all the time. Having been there, I would not.
PS: your boat would be subject to impoundment and you to severe fines at minimum if you were found to have visited. I don't mean to sound paranoid, but there are these spy satellite thingies...