While current US government regulations do not require a US passport for travel to or from the US Virgin Islands. However, we recommend you bring a passport (or you will need a certified copy of your birth certificate and, for anyone 18 or older, a driver's license or other government identification with a photograph) in case you want to visit the British Virgin Islands.
When you leave the Virgin Islands, you will need to clear US Customs in the St. Thomas airport. While a passport is not necessary, it does make things go faster.