Island Supermarket is very expensive and make sure to check the sell by dates. It caters to people who really want food from the USA and can't live without it. There are other excellent supermarkets on the island that do basics for a much better price.
There are a number of wonderful places to eat coming up from the South. Sandbar (Fido's 2), Victoria House (ate a wonderful meal there this week), El Divino, Antojitos (local food), several Chinese takeaways, Ricos, George's Kitchen, Pedro's for pizza and many others along the way. You'll have no problem finding great food. Make sure to try the small local places, holes in the wall and vendors (there is a wonderful tamale vendor who cycles down south) who all do really great local food at good prices.
As for the topless situation, it is not considered acceptable. If you are discreet and sensible or able to guarantee your privacy, then go for it. We quite often have people skinny dipping in our pool at night despite the fact that every unit overlooks the lit up pool. Just remember that you are in a Belizean country, not an American one. Therefore it is the Belizean sensibilities you need to consider.
Interestingly, I have seen pictures of Maya women in Southern Belize in the 60's and all of them went topless (though not for tanning). I suspect the missionaries put paid to that.