This is an update from March 2016. The ferry company mentioned in the other answer now states:
[...] as January 2016, ferry operators are still awaiting final approval and licensing from the Cuban government. Because of this, CubaKat (and other ferry companies) are waiting to begin full operations.
CubaKat’s goal is to offer our ferry service, from the Florida Keys to ports within Cuba, some time in 2016. Currently, we’re working with officials from both countries to make this venture a reality.
Whereas at least the second statement sounds more like a marketing statement than a reliable prediction.
The Miami Herald run an article on this last month, and wrote the following
Nierenberg now expects U.S. ferries to be sailing to Havana by late 2016 or early 2017.
quoting and official from another ferry company.
Overall it seems like there is no official answer yet.