Sea of Thieves fully released March 20, 2018 on Steam for PC (Windows) and Xbox One.
You could pre-order the game for $59.99 USD (prices may vary between retailer), which offered access to the Sea of Thieves Closed Beta (now closed), and offers exclusive in-game items for the full game.
Alternatively, you can purchase an Xbox Game Pass subscription for $9.99/month (plus tax), which will grant you access to Sea of Thieves[1] (as well as other games available in the Game Pass library). At this time, Sea of Thieves is intended to be a permanent addition to the Game Pass library, per Larry Hyrb (citation needed).
Sea of Thieves is an Xbox Play Anywhere title, meaning you can play on Xbox One and / or PC. This also means PC players can subscribe to Xbox Game Pass for access to Sea of Thieves as well.