Just got back from Costa Maya 2 weeks ago. Order your groceries from Simon at Lagniappe Provisioning and eliminate the hassle of transporting your groceries either by water taxi or golf cart. Simon does a great job and it was so nice to have everything in our unit when we arrived. Grocery prices are much higher than in the US.
We were able to get a weekly pass with Island Ferry for $50 per person for the week. We also rented a golf cart for 2 days just for the adventure. The Island Ferry is the better option for getting back and forth to town.
The onsite restaurant at Costa Maya was quite good. I didn't restaurant prices to be too bad at all. There are some local places in town that are very inexpensive.
I highly recommend Belize Academy of Diving. There were four of us in our group, each with different skill levels- I did my certification dives with them, DH had some experience and Bob & Susie each had over 150 dives, and all of us were very happy with them. You didn't say when you were going but they will be closed for the month of September.
You can see my pictures of the resort at http://www2.snapfish.com/thumbnailshare/AlbumID=171268364/a=103724515_103724515/t_=103724515
Have a great trip!