Enjoy
Certainly you can usually save money by going with a private tour guide but if these are you first visits to these ports, I'd go with the cruise excursions. It's hard to put a $ amount on the peace of mind knowing that the ship will be waiting for you if your tour is delayed. It only happened to us once (in Alaska) and the ship waited 2.5 hours for our tour train to return. On occasion, when we've been somewhere before, we'll strike out on our own.
Assuming no named storms, October is a great time to cruise. In fact our close friends did get delayed by a storm in October several years ago, but they weren't complaining since their 7 day cruise became a 10 day cruise and returning to work wasn't a huge issue. Glory is a big ship and rides well regardless, unless your cabin is in the very front or back. We once had a balcony stateroom on the back of the Conquest (sister ship) and it was great until the last day when we were heading into a big sea. That was not a "fun day at sea".
Been to each of your ports several times:
Cozumel is easy and fun...favorite activity was a combo SEGWAY/snorkel tour. Although Mayan ruins of Tulum are interesting and it's a beautiful spot.
Belize is the place to snorkel and the Mayan ruins are good.
Costa Maya is still establishing itself as a port. Shopping and drinking are the activities there. Keep your hand on your purse/wallet in crowds.
Nassau is Nassau...not sure if a ship excursion is necessary there. In the past, we've grabbed a cab or the water taxi over to the Atlantis and finished with a walk through town.
In short, there really is no better time to cruise than October. The price is right, the air is cooler than the Summer, but the water for swimming and diving is warm.
