There are several possible itineraries for the Cancun Havana cruise. At present the three day itinerary leaves Friday morning, allows passengers ashore in Havana from around 11:00 to 19:00 on Saturday, and returns Sunday evening. The four day trip leaves at midday Monday, allowing passengers a night and day in Havana, then returns by Thursday.
It is also possible to remain in Havana for a period of a week or more, returning on a later boat. This, or course, has to be scheduled with the company when buying tickets.
Current prices can be seen and reservations made at the company's website, but they start at around $189 dollars for the 3 day trip in the cheapest cabin. At this writing they are offering a two-for-one deal, which is not unusual in the slow summer months. There are additional costs involved. Passengers pay port fees, $40-45 US dollars depending on exchange rates. There is also a "drinks bracelet" to buy if you want to drink anything. This ranges from around $30 to $60 USD due to a complicated tier of national or imported drinks allowed (or for bar service, but not including alcoholic drinks). Bracelets allow unlimited drinking at meals of any of the four bars. Gratuities are not included in the fare (and richly deserved).
For comparison, a Cubana flight from Cancun to Havana runs around $200 USD, but arrives at (duh) the airport, which is way the hell out of even Havana's sprawl and involves exorbitant and time-consuming transportation into the city--possibly costing as much as $80 USD. The Boat, on the other hand, arrives at the Malecon--you just step ashore and are deep in Old Havana, facing old stone buildings, horse-drawn carriages, and streets leading directly into the heart of things.
Transportation from the dock is widely varied. There are state cabs, safe and expensive, to take you anywhere in town. Little "coco" cabs are at least worth a photo: they are orange fiberglass globes seating three people, looking like oranges or football helmets rolling around. Gypsy cabs are cheap, offer no guarantees from anybody, but can be super cool--what price cruising in a 1950 Chevy or 1962 Studebaker? There are also antique horse carriages for stately clip-clopping through the old stone streets, and even human-powered pedicabs.
Foreigners need to carry a valid passport (you will have one by that point--the ship requires them for boarding and hangs on to them during passage). You fill out a tourist card but the system, from ship to Cuban immigration is careful to segregate and tag American passports to avoid saddling US passengers with a Cuban visa stamp. In view of the recent Bush "crackdown" on Cuba visits, this is a wise precaution, and paranoia about proofs of visiting Cuba runs high--possibly because of the rattling of $55,000 fines. Don't worry about it, but make sure you are complying at all steps.
And DON'T traipse into US customs with Cuban cigars. They used to just destroy them, these days you could actually get into trouble--at the very least a delay and major hassling. Cigars without labels, by the way, are ASSUMED to be Cuban and destroyed. Illegal and unconstitutional, or course, but what are you going to do?