Welcome to CruiseCrazies, dacsmom.
My wife and I did the cave tubing in Belize in April. We booked it through the ship. It was a pretty good excursion, but will be much better in December. April is the end of the dry season and the river is very low at points. This should be about the beginning of the dry season, so the river will be nice and high. As it turned out, the ship's excursion went to the exact same place as a privately booked excursion, so there was absolutely no difference. The worry of missing the ship is always enough for mee to book through the cruise line. It is a long excursion. Make sure that you get a bottle of water before you leave the ship. Enjoy. :cool: