If you don't mind the cost, buying water by the case is the safest route. Trois Pitons is the best Dominican company, and the price is reasonable compared to Blue Mountain, which is imported from Trinidad.

The smartest thing to do is to save your water bottles, keep them clean, and fill up at the campus water fountains. The 0.5 micron filters on the fountains is better than Brita and you can be 99.9% sure that there aren't any unsavory organisms making their way past the filters. Keep about 10 bottles and fill up every weekend. You'll have plenty of clean filtered water for the week.

Yes, it is a little more work and time-consuming lugging all that water back home. But you're in a third world country and you're lucky you don't have to boil and filter the muddy tap water yourself...!