Welcome to cruise chat hotcruiser!!
You are correct about Carnivals cave tubing, they do just let you free float out there. You will find that the independent tours hook the people together and lead them through the caves, plus they have more guides for the amount of people. Also, most of the independent tours take through more cave systems then the tours with Carnival.
I agree though, that anyone that has a problem walking should not due this tour..it is a rough walk. And remember these tours are on a time schedule to get everyone back to the ship.