How is the drinking water on St. John? Is bottled water required?

No, bottled water is not required. The drinking water is potable, but it can taste different from the water you are used to at home. I prefer the taste of bottled water, so I picked up gallon jugs of water at the store to refill my 1/2 liter bottles for day use. Cooking, toothbrushing, washing all were fine with tap water.

I wouldn't drink the water. It's rain water collected from the roof. I boil pasta in it, take showers/baths and wash dishes and vegetables in it but you should play it safe and just buy gallon jugs of water at the supermarket for drinking.

Its safe to drink, but I normally drink bottled down there only because I do not care for the taste, never got used to it living in the USVI

I drink bottled water, and use tap water for bathing, brushing teeth, and washing dishes.

We used the cistern water on St. John for making ice tea with no problems. The faucet did have a filter attachment. We've also used cistern water in Bermuda and the Abacos with no problems.