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 (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.