Sea of Thieves will be released on both Xbox One and Windows 10 in Early 2018. The window was announced today at Xbox's E3 press conference.
At E3, Rare showed off a Sea of Thieves mission that began with a a group of pirates readying their ship for sail. They then swamp down to a ship wreck and plundered it for treasure before being chased back to their boat by a school of sharks.
Back on the mainland, the group explored a cave and solved some light puzzles while attempting to find a secret treasure. While one player carried a chest (thus being vulnerable to the group of skeleton enemies hot on their tail), the others had to protect him using swords and guns.