We went in April and it was one of the best excursions we have ever been on. We booked through the ship.
I started talking to our guide during our walk through the jungle. He explained just how dangerous it can be after it rains and how the river paths change all the time during the rainy season. If it rains at all, they go back into the caves and map everything out. They do this each time it rains, that's how important it is.
Now, I would never take a young child or 80 year old, even if the water is totally calm.
When we went the water was between 1 inch and 40 feet. The river was as calm as could be so we had to paddle with our arms to really get anywhere. The only way you could ever get our of your tube was if you climbed out, not from the water.
That said, if I ever saw a wild river, I would turn around. Never would it be worth risking your life.