I was on the cave tubing excursion the first time Carnival stopped in Belize back in 2001. This was in November so it was the rainy season also. The guides had just macheted the trail through the jungle leaving small stumps that people were stubbing their toes on. People were tripping and falling during the walk. There were broken ankles, toes and sprains.
When we got to the river, we did not pair off, nor did the guides tell us what to do. There were no life vests available either. We just got in and went for it. After we got through the cave, a couple of us got caught up in the current and were pulled into some thorn bushes. The guy in front of me had his shirt ripped off him by the thorns, I saw myself going in right behind him. When I ducked to try to avoid the thorns, my tube was punctured and deflated. The water was deep and I wasn't able to touch, so I had to swim over to some other tubers.
After we got out of the river, I had cuts on my arms and a thorn in my butt that had to be removed. The other guy had cuts and blood was running right down his back.
This excursion is rated as Very active excursion involving constant effort. The excursion may require continual physical movement, including paddling, walking long distances, or climbing steep inclines.
I would never take children or older parents on this excursion. I know ours was unusual as it was the first one and we were the guinea pigs for it. We did get half of the cost reimbursed on our S&S cards because of all the trouble and injuries.
So, I know exactly what you're saying about cave tubing. Be careful and do research on excursions.