Buck Island is best visited by day trip from Christiansted, St. Croix's waterfront capital. A company called Big Beard's Adventure Tours runs sailing trips on its two catamarans: Half-day trips depart on a 40-footer named Flyer while full-day sails are aboard the 42-foot Renegade, and yes, that's the Big Beard.
Depending on sea and wind conditions it'll take anywhere between 30 and 60 minutes to get to the island. Once there, you'll be treated to one of the most diverse underwater ecosystems anywhere in the Virgin Islands: Elkhorn coral formations, deep grottos, sea fans and schools of tropical fish including blue tang, angel fish and more.
All snorkeling gear is provided and you can buy drinks and snacks on board, so pack light. And if you've got an underwater camera, this is definitely the time to bring it.