Don't know if you have booked yet. There is definitely more competition out of Miami & Ft Lauderdale because of the sheer amount of ships ported there in December. Hopefully you chose that week because you are travelling with kids because most ships have at 30-40% occupancy of kids/teens during vacation time (christmas break, spring break, easter break) and if you're not travelling with kids yourself, it can really change your level of enjoyment (not necessarily for the better).
Hope it all works out for you.