You can do one of several things. You can buy a phone card and use one of the pay phones. All phone calls are very expensive. All water from the taps has been treated and is considered drinkable. In actuality it depends upon where on the island you are staying and how much of a purist you are for taste.
Bottled water is readily available for sale but can be expensive.
There are numerous shops which carry the necessities like toothpaste, toothbrushes, shampoo, sunblock, etc. However, you will find things much more expensive than your home drug store so stock up before you come. There is a clinic staffed with English doctors on Provo which can take care of basic needs. For serious emergencies you would need to be flown to Miami. Provo has a decompression chamber on island and a doctor is always on call.