We did the cave-tubing/zipline combo with Major Tom in 2007 while on VALOR, and to this day it's one of the most exhilirating things I have ever done. Major Tom and his outfit are complete professionals, and from what I understand he is a pioneer in the cave tubing experience.
Major Tom met us himself on the dock after we had tendered into Belize City and then we rode a lengthy period to the excursion site. At the park we hiked for 45 minutes into the rain forest while Tom talked about the native plants and animals and the lives of his ancestors. We got the chance to stop and talk about poisonous snakes (don't get off the path), ancient rituals (there were magic mushrooms on the wayside), and an ancient Mayans diet. We all got the chance to eat huge termites from a fresh nest...Tastes like Mint!
Once we got to the river, we got into our inner-tubes and joined up into connected groups of 12, as was already mentioned.....Tom, or one of his sons, steered us throughout the excursion and provided explanations of what we were seeing and how best to navigate the river. We were never in any distress, though we saw tubers from Carnival and other cruise lines being swept away by side currents or into the brush areas because they were left free floating rather than having a cruise line representative help them.
In the end, after both excursions, our only concern was getting back to the ship on time...and this is where I need to caution everyone.....we almost didn't make it back to the ship!!! We were the last two people to walk onto the ship before the door was closed to sail away.
I credit that near miss entirely to Carnival.
Our excusion with Major Tom was to include passengers from both Valor and Legend. Legend was due in port an hour before us...However, a passenger suffered a heart-attack onboard after leaving Cozumel, so the ship turned back. She arrived almost 2 hours late to the port...which skewed everthing for the whole day, given the distance to the locations.
I'd book Major Tom again anytime. He's a great host with a great knowledge of the area. Be more aware of the ships.