we go to all inclusive resorts in mexico every year and cruise as well once a year so it can be harder to stay on plan.. but here is my theory you are on vacation.. don't be so consumed in calories counting. it's a break! you can make healthy choices and not go overboard but i wouldnt stress about exact numbers. leaveing things liek sour cream, cheese, and gauc off are great little calorie savers! breakfasts will be easy because almost every resort I've stayed at has a breakfast buffet full of fresh fruits (and they taste better there in mexico soo fresh)cottage cheese, oatmeal, whole grain breads and muffins, omelets made to order, etc.
and i know they will have salads on the menus for lunch and dinners. seafood and chicken is always good there!
if your alcohol is included too I say go fro drinks like diet coke and rums. or take a few straight up shots!

or even ligth beers.
but again honestly I say eat and drink what you want...but WITH IN REASON.. just make sure not to gorge yourself...
the other great things about vacation is I'm typically more active with being in the pool, snorkeling, playing volley ball, hiking and exploring,dancing etc.. your going to burn a lot of calories just being active!
have fun and I'm super jealous I wish I was going! where are you going? my favorite spots in Mexico is the cancun/ playa del carmen areas in the yucatan! that entire area is paradise!