Rather than eating out EVERY meal, get a condo/apartment and make your breakfast and lunch with a dinner or two out on the town. The Cultimara is not the cheapest food on earth, but if you stay with staples, and away from things that are hard to get on a small island you can shop on a budget. Bring your own spices or your food will be bland or your budget takes a hit. Keep in mind all of the prices are in NAF so it always seems expensive until you do the conversion. The 'farmer's market' is at town pier on wednesday and ??? (brain freeze) and you can get good fruit on the cheap as well.
Tap water is good to go, so stay plenty hydrated during the day and you can even pack your own gatorate powder along with you if you want.
I personally think the worst part is airfare (at least compared to other caribbean destinations). $600 - $700RT seems like the going rate year round. I have a trip in June that I got two roundtrip tickets for $280 (for both) plus FF miles.
If you are there soley to dive, the cost per dive is less than anywhere else I have been to in the tropical world (as long as you are shore diving).