I arranged all my excursions in advance except for Santo Thomas which was good because we went to Roatan Honduras instead due to problems in Guatemala.
In Costa Maya to visit the Mayan ruins we used The Native Choice (Ivan Cohuo responded to all of my many email inquires in a couple hours or less). I Highly recommend this company http://www.thenativechoice.com We took the excursion tour of Dzibanche which also includes Kinichna (it is an acropolis that was part of Dzibanche - you can find great background information on all the available ruins using Google). There were 5 of us on this tour in a new full size Van with a driver and a tour guide. She gave great detailed information on Mayan history and culture which made the 2 hour ride go by very quickly and comfortably. When we got to the site there was just the 7 of us and we latter saw 4 workers making repairs. We also latter saw a couple of other people who were exploring on their own - not from any ship. Incredible site as we were able to climb up to the top of 3 Mayan temples. We made it back with a few minutes to shop before boarding the ship. Just what I was looking for!
In Roatan Honduras we wanted to go scuba diving although neither of us had done it before. The ship offered something but not enough people signed up and the trip was canceled. When we got off the ship we went to the information counter by the pier and the girl there said nothing like that was available. We then talked to someone else who told us to walk a little further and turn right to see all the available private excursions. We went to the last information booth on the left and the guy there (Mario) set us up with the GREATEST Dive Shop www.anemonadivers.com. He arranged the taxi to get us there and back and the dive for $99 each. I paid him and gave a white piece of paper to the dive shop to show we had already paid. We met the two women who run the Anemona Divers and had an incredible experience. These two women have their hearts and souls dedicated to sharing the joy of scuba diving with everyone. Even though I had never dived before we were given about 45 minutes of instruction and after proving we could clear our mask of water and retrieve our air hose if we lost it we were able to go diving. First at about 3 ft, then 5 ft, then 10 ft and finally at 20ft. Unless you looked up to see the surface right above you it felt like we were deep in the ocean. Sara always made sure we were OK by asking us to give her the OK sign and she took pictures of us and of what we saw (a sea turtle, lobster, and lots of beautifully colored fish). It was an incredible experience - Thank you again Sara!!
In Belize, We enjoyed an unBelizeable cave-tubing experience with email@example.com. Vitalino is a wonderful guide and treats you like family. Our tour with him included 10 more minutes of walking and an entire extra cave to float through. When I was taking pictures on the hike Vitalino took my inner tube and carried it so I could take pictures with ease. He had a dry bag he carried with him so things we wanted to keep dry did. When the water was shallow or slow he would hand row us or walk us through the low spots so we did not need to row. Same park and caves as the ship excursion but so much better and personalized with Vitalino. Contact him and you will be happy you did!! Great guy and great family! He even had sodas in glass bottles for us in addition to lunch.
Last stop was Cozumel. I arranged in advance on the internet to go to Chankanaab National Park and take the Royal Dolphin swim with www.dolphindiscovery.com The ship tour does not include the Dolphin push where you rest on your stomach in the water and the dolphins come up behind you and push your feet with their noses until you rise up out of the water like superman taking off. It is amazing!!! I suggest buying the 3 pictures and the video from them as they do a great job. I took pictures with an underwater camera and it distracted me from the joy of the entire experience. I booked in advance on the internet and received discounts on the Royal swim and the DVD/photo package. Great experience but our "guide" couldn't keep track of anything. Trainer was great - I suggest you do what feels right and natural and enjoy your experience. Make sure you look good wet before getting in the water to get the best possible pictures. You will want all the pictures and DVD after you see them. Dolphins are much more massive and dense than I ever thought. Easy taxi back and forth to the park $11 flat rate posted at the taxi stand.
I put a lot of effort into finding these excursions after reading hundreds of reviews by other travelers. We ALWAYS felt safe in all we did. Happy Adventures!!