Sea of Thieves will be a 'multiplayer adventure game' is how it has been described in various reports. How 'massively' multiplayer this is, remains to be seen, it could be servers containing limited numbers of players or it could be one huge open world. We don't know yet.
It will be players, playing together, sailing ships, fighting, looting, plundering and all the things pirates love to do.
There is also a lot of vague suggestions about some way in which players can craft their own story within the game, lots of player choices this sounds like.
This is about the most recent description of actual gameplay and it definitely is multiplayer.
http://www.gamespot.com/articles/rares-xbox-one-pirate-game-sea-of-thieves-gets-new/1100-6434271/