There's frequent daily service from both Red Hook and Charlotte Amalie on St. Thomas to Cruz Bay (the more frequent ferry is the one from Red Hook). There's also frequent service from Cruz Bay to Tortola and less frequent service to the other British Virgin Islands, including Jost Van Dyke (some via Tortola). Virgin Islands ferry schedules are published on the website of the Virgin Islands Vacation Guide & Community. The actual schedules change, so you should check with the ferry companies to determine the current schedules. Remember that a passport is now required to travel between the USVI and BVI by ferry.
Virgin Islands Vacation Guide and Community. www.vinow.com.