Thanks for your replies guys. Florida to Havana via Canada seems a bit extreme ...
I think I've come up with something workable - unless there are regulations/restrictions I don't know about.
MIA-CUN - AA - using BA avios 4,500 + GBP 16.40 taxes ( + USD 25.00 hold baggage charge)
CUN-HAV - Interjet - currently at USD 104.00
Because of the ETA of the cruise into MIA, it does mean an afternoon flight out of MIA which then necessitates an overnight in CUN. Is it worth a shuttle to the beach for the evening from an airport hotel ?