For starters, I'll recommend that you relax! Mexico has some absolutely wonderful photographers that take stunning photos. It will come down to your budget, what the outside vendor fee is at your resort, and what you're looking for.
Here are the ones that I've looked at, which is probably most of them!
www.delsolphotography.com Very popular with brides on this site. For my budget, they were high but that is strictly a personal choice.
www.cancunphotos.net Sascha Gluck too does some beautiful work, and also responds quite quickly. I'm still looking at him for us.
www.deansandersonweddings.com Dean and his wife Anna seem very personable, and she answers inquiries very quickly. She also provided some suggestions for some things that I was trying to get information on. I really like his work but for my budget I'm still trying to figure out if I can afford them.
www.bicoastalimages.com This is a couple that do photography. I really like their work and they are also on my short list. Their photos for what I want are really great!
www.ceciliadumas.com Cecilia's work is stunning, at least for me. Her colours are fabulous and she is on my short list.
I would highly recommend going through the vendor reviews and see the different comments from other brides. And check out their websites to see what attracts you to their work, if it actually does. Everyone's tastes are personal when it comes to photography so you'll see something in one that you like, but maybe not in another. And as I said, your budget will be an important factor plus how much your resort charges to bring in an outside photographer.
This isn't all of them by any means - it's just some of the ones that I looked at that stuck with me when I reviewed their photos.
Good luck and happy planning! Oh yes - and welcome to BDW. You'll definitely find enough information here to get you through the rest of your planning!