Linda, Norwegian has an excursion to ISla Mujeres. (ISland of Women) The tender ride is a bit long but the Island that awaits you is well worth it. You can walk into the ocean for two hundred feet without going over your waist in some of the most beautiful water you will ever see. You can snorkel from the shore as well. Saw a few stingrays in water that was about four feet deep. There are chairs and umbrellas on the beach, and a restaurant as well. One thing you will not want to miss is a place called "Pool of the Gods?" You will stand in a natural pool right behind the resort, and look down to see around two hundred fish of different colors right at your legs. (It may be called Bath of the Gods, can't remember.) There is also a shopping area just off the resort grounds that has great shops and faster restaurants than the one on the beach. They rent golf carts to go into town and you may want to consider doing that. At any rate, the kids of all ages will love this place. It is like walking into a travel poster.
I know that there were some folks on the tender to the island who were not from the ship. I do not know how much they paid. I am sure you can arrange this on your own. The ships excursion was around 37.00 each and included all you can drink. This of course was a big draw for some. Maybe someone else can help you to find out where this can be booked.