Well, the Coast Guard has spent considerable time, money, and effort learning how to effectively detect and (if necessary) stop people in boats from getting into the US from Haiti and Cuba- at this point, they're pretty good at it.

Not to say that with sufficient funding and time it wouldn't be completely possible; just that it's not as easy as setting out from Havana and ending up in Miami.

Sneaking into the country's no picnic, no matter how you do it. I've always thought that this fact says a lot about what the places that people who do sneak in must be like.