My DH did this twice through the ship and loved it, but Carnival no longer offers it. I did not go because I would be unable to do the 45 min. walk to get to the river. I would never do a zipline, as I am terrified of heights. I would literally have a heart attack.
But, one thing people might want to be aware of is that the river is not exactly pristine. Any open water anywhere can have contaminants that you can't see, but this one has some nasties that are ONLY a concern if you have an open wound. No problem if you don't, but if you have a cut or large scrape, either don't go or make it waterproof, either with waterproof tape or something like Nuskin would probably work. And, do NOT swallow the water!
I don't mean to scare anyone off as this is a fun tour. Just use some common sense to stay healthy.