I think the ceremony location is based on which part of the resort you're staying in. If you're Akumal, then it's 1 or 2 specific locations you can choose from. Check out this thread here. Trisha has photo tours of the resorts and some of the wedding ceremony locations.
http://www.bestdestinationwedding.com/t/75549/official-gran-bahia-principe-riviera-maya-thread She also has a lot of other information on there, that might help. I'm staying at the Tulum so I believe my wedding ceremony will be held at the gazebo. I could be wrong, but I'm just going to wait for now, and plan my other things for now. Since I don't know the number of guests attending, I'm just brainstorming ideas for OOT bags, my ceremony programs and scripts, and deciding on the centerpieces for the reception (since I will most likely choose one of the restaurants at the resort).