Theoretically this ought to be possible - it's a relatively short hop across the straits from Cancun to Havana but I doubt that it would be practicable or affordable. There are scheduled flights from Cancun to Havana but on a brief search I can't find any that would make it feasible to get there and back in the same day. Also, they've come up at around $300 US on top of which you would need to add 25CUCs for your entry visa and another 25CUCs for your departure tax which would roughly add another $50 US on top. I am not aware of any ferry service across the straits but at an approximate distance of 100miles, even if there were, the return journey wouldn't be 'doable' in the same day.

Old Cancun 'hands' might be able to point you in the direction of a local excursion operator who does organised day trips based on a charter flight to Havana but it sounds unlikely to me. Probably far better to book a Cuban holiday some time in the future and do a trip to Havana as part of it - especially as one day is not really enough to do such a wonderful city justice.

SM

We did a two night trip to Havana flying with Cubana, booked with a local travel agent in downtown Cancun, as it was over 10yrs ago I can't remember the price, but at the time it seemed reasonable enough. Hotel and full day sightseeing included, I'm sure these sort of trips will still be available, some Americans were doing it, making sure not to get their passports stamped !
x lassi

We did a two night trip to Havana flying with Cubana

Whilst one part of the Cubana website indicates that it is possible to fly with them from Cancun to Havana, it doesn't show up on their route map and a search for flights on that route never returns any results. I suspect that this means that they no longer operate direct Cancun-Havana flights. And as usual, that they've not updated their website properly!

SM