I don't want to discourage you, but you do need to be physically fit for this one. Carnival stopped offering it as a tour after 2 people died last year. Now, my hubby has done it twice and loved it. I couldn't even tackle the walk from bus to the river, so I didn't try. But, there can be very strong currents and even he admitted that he once had to rescue a woman who was being pulled down a current the wrong way. Luckily, he is strong enough that he was able to grab her and paddle the two of them back into the main channel. The other. more minor, caveat is not to let the water get into your mouth and make sure that any sores or cuts are not exposed to the water (I would put a heavy coat of ointment under any bandage to waterproof). But, some nasty bacteria in the water.
But, all that said, he would do it again....and again...and again. And I would, too, if I could. He said it was truly awesome.