We have stayed at Costa Maya before. You will have to buy fruits and veggies in town before catching the boat that will take you to the resort. We bought the weekly pass for the water taxi, which gave us unlimited trips into town. The taxi runs every hour except after lunch it runs every other hour till 4PM. This way we came into town and bough what we needed.
The baker is open 7AM -7PM every day on front street for the best baked goods. Green house has great fruits and vegetables and even groceries on middle street. Superbuy on back street has great groceries on back street. They are all walking distance from the water taxi terminal.
The restaurant at Costa Maya did not open till 8AM and closed at 8PM and the food and service was mediocre at best.
The groceries they sold was expensive and not fresh.
Here is a map of AChttp://ambergriscaye.com/pages/town/mapdowntowndetail.html