Typically you will have $10 per day per person as gratuity on your sail and sign card. The carnival web site will give more detail. The rest of the bill depends on how much you drink, use the paying part of the Spa and charge stuff.
I have flown to Ft Lauderale and sailed Miami twice. Both times I used a Comfort Hotel the night before and booked the room to include transport. I recall it was about $12 each doing it that way.
You will love Cozemel. One of my favorite ports. I hope the dock is repairing well, it was destroyed in one of the hurricanes but I understand it is partially rebuit now.
Be absolutly sure to go to the Cruise Directors briefing on shore excursions. Ignore all the hype about buying stuff but he will tell you good things to know about saftey and taxis and such. Some places will have taxis that are just people trying to rip you off. The CD will tell you how to tell the difference between real taxis and just a guy with a car.
Caribbean Activities & Attractions