The first big Sea of Thieves update hits today. Free to all players, Cursed Sails adds skeleton ships (finally, ships to fight that might actually have loot on 'em), player alliances, as well as the Brigantine, a new ship built for three players.
The latter addition is the most exciting to me, as the Galleon is a bit unwieldy with three players, which also happens to be the largest party I can usually get together. The Brigantine wasn't originally planned, but player feedback set Rare on the task—you can learn more about it in this dev update video.
Also coming today are player alliances, which allow you to split loot with other crews. True to Sea of Thieves' competitive, backstabbing nature, though, alliances can be broken. Check your signal flags in the crow's nest to start forming alliances—you'll find new triangle-shaped flags that allow you to offer and accept truces.
There's also some minor bad news. Due to performance issues with the Cursed Sails update, Rare has temporarily disabled skeleton forts and the kraken, opting to get the big update out on schedule rather than wait for the problems to be solved. They'll be brought back "as soon as possible."
Update: Cursed Sails is available now. To force Sea of Thieves to update in the Microsoft Store, click the '...' dropdown in the upper-right, head to 'Downloads and updates,' and click 'Get updates' (so intuitive!). At the moment, we're having trouble matchmaking (error: Cinnamonbeard).
Update 2: After many failed attempts, I've made it into a game with the new ship. Looks like the issues are being ironed out.