Went cave tubing in Belize last week on a shore excursion with Royal Olympic's Olympia Voyager (If you don't know about ROC, check them out. Greek cruise line with 2 new smaller but fast ships (28 knots) one in Houston doing West Carb and one in Med doing Rome, Venice, Athens.)
Anyway about cave tubing. We paid $55 and that included the bus, a guide, lunch, admission fee, and a snack on the bus for the return trip.
The tubing part was about 1 hour and 30 minutes in the water. You bus inland through Belize City and into the countryside for about 2 hours. The last 6 miles are killers on a bumpy road. The trip inland gives you a chance to see and learn about Belize from your guide. Not sure I would do this with a taxi driver because of the distance inland. IMO a formal shore excursion is the way to go.
At the resort they have changing rooms (small and the floors get wet quickly). The guide usually will provide an area to secure and guard your belongings. When ready you take your group will each take a tube and follow your tubing guide on about a 20 - 30 minute walk through the jungle. You are basically walking up river to where you will start tubing. The walk is interesting. We saw army ant hills, hornet nests and heard rustling in the vegetation (the guides said they was critters watching us).
The caves are big and dark but everyone has a headband with a flashlight attached so you get lots of shadows and reflections of light off the water.
There are spots where the water is deep with very little current so you have to paddle with your hands. There are also shallow spots where you have to push off the rocks and lift your bottom out of the tube to get through the shallow water. The people who seems to have the most fun were those who chained together for a group effort.
All in all it was fun and a very unique experience.
We used the ship shore excursion process. Most of the ship that stop at Belize offer this one. It is very popular. The giant NCL Sun was also in port and there was a grundle of people from that ship cave tubing also.
If you check destinations for Belize on Cruisemates and Cruise Critics boards, you may find other sources.
Our feeling was that where the tour was not that expensive and included lunch ($55) that we could use the ship's offer. There is some security in going on the ship's tours, especially if there is a lot of inland driving.
As always feel free to send questions directly to me via email.