I just returned from the 7/5 Magic and would strongly recommend AGAINST doing Tulum on your own. You will spend 2 hrs. getting there and the same time getting back. The travel involves a water ferry to the mainland and then a long bus ride. There are too many variables and too many opportunities for something to go wrong if you're on your own. If you do it with Disney, you'll be with lots of people....we had several busloads....and if anything should go wrong, at least you won't have to fear getting back to the ship on time. You may be able to find a cheaper price elsewhere, but I would strongly caution against doing it with another carrier.
On the other hand, we did Stingray City on our own with Capt Marvin's and that was a great experience. The difference with this excursion is that it's located only 2 blocks down from where the ship docks so there's less liklihood something will go wrong. Capt Marvin has over 50 years in the business and is a real class operation so I wouldn't hesitate to recommend them, but there are lots and lots of other companies (?) soliciting your business as you step off the pier and they don't appear to be as reputable.