Six additions were announced for The Sea of Thieves to come out in the summer of 2018. The Hungering Deep was the first and this came out in May. On July 31st, well-timed for children with the whole of August to find amusement for, Cursed Sails and Forsaken Shore were released together with a completely new ship.
DLC is extra content for a video game that enhances or completes the video game's features. The additions mentioned were free. It remains to be seen whether the rest will have to be paid for.