not sure how the water is at that time of year but when we went it was so shallow in some places that we sraped our butts on some small rocks, no injuries cuz we had plenty of warning, it was just a bit unplanned. If you feel confortable with an inner tube under you , you should be fine. I can't inagine the water being more than 5 or 6 feet deep.
The $80 is about right. Actually ours was much more , we had to pay for air fare also from AC. For us that included the cave tubing, the zoo and lunch at a place called CHEERS(In Belize??)nothing like the Boston place but they have an incredible display of T-shirts that people have left for decoration. If you have a unique one, leave it there for all to see.