Cancun is a weird place.
Six months is a long time. You might want to try to sneak in a hot plate or a microwave. This way you can make instant rice or something, or heat up some vegetables.
Cans of tuna, are probably the obvious choice to ship. Cans of chick peas, protein powder may also be good to ship. Apples can last 6 months is a refrigerator, but it's probably better to just stick to the local produce. Milk, yogurt, and stuff like that is easy to find there.
I've traveled for extended periods of time in foreign countries, and best way to stay healthy is eat what the locals eat. In cancun that's rough, because it's this fabricated place. Look for a supermarket when you get there. I'm sure you will be able to find everything in cancun that you can find here.
Here's a list of some supermarkets:
Cancun Supermarkets : TravelYucatan.com