The Hungering Deep is the first DLC for Sea of Thieves. It was released on May 29, 2018.

This content update introduces and showcases a raft of new features that will persist in Sea of Thieves beyond Merrick’s story and its time-limited campaign. (From the patch notes )

Bilge Rats – To stay in the know as to what is changing in the Sea of Thieves, visit the Bilge Rat character in the taverns to discover what time-limited events are active. Whether it’s The Hungering Deep or future events, this is the place to go!
New Threat – Something old, vast and hungry is surfacing. Follow the trail of riddles and rumours. We hope you live to tell the tale.
Speaking Trumpet – Parley! Let other ships know if you are friend or foe with the new Speaking Trumpet, making yourself heard from longer distances. The first Speaking Trumpet is obtainable through The Hungering Deep campaign, but purchasable variants will be arriving in the shops in the near future. (To get the first one, talk to Merrick and ask him how he lost his legs.)
Ship’s Flags – Want to sink and plunder, or seek allies to adventure with? Let your intentions be known with new flags which can be raised high above your ship! Climb up to the crow’s nest and decide which to raise.
Drum – BOOM! Pirate percussionists (inspired by Merrick himself) can now be heard all across the seas. The first drum is obtainable through The Hungering Deep campaign, but purchasable variants will be arriving in the shops in the near future.
Tattoos – Many pirates wanted them and now they are here! Check out the vanity chest for 16 sailor tattoo sets and visit the clothing shop on outposts for stocks of four purchasable tattoo sets.
Scars – What enemies have fallen to your pirate might? Scars earned through campaigns and events such as The Hungering Deep will allow you to tell a story of your pirate’s history and adventures. These are not purchasable, they must be earned.

Guide
Sea of Thieves – Fin rock on Shark Bait Cove
Travel to Shark Bait cove and on the South West on a smaller island is ‘Merry’ Merrick, playing his drum. Talk to him and ask him how he lost his legs. He will gift you a speaking trumpet, for communication with other crews over long distances. This will be important later. Once you accept his gift “The Hungering Deep” will begin.
He tells you to go to find his journal. Find it on the altar in the centre of the island. Return to Merrick after reading it. Merrick mentions a tavern barmaid at a shining place. He is referring to Tina at the Golden Sands Outpost. Talking to her is not required, but players that do willl learn about Merrick’s ship, the Killer Whale and the location of the wreckage.
You now have to go to two uncharted islands (approximately L-14 and S-16). The first island will have pieces of Merrick’s ship The Killer Whale scattered around. Look for the mast and climb into the crow’s nest. The journal is in the nest. The second isle (S-16) will have a sunken temple close by. Once you find the temple archway underwater, swim straight down. Follow the completely submerged tunnel into a cave. The journal will be inside leaning against a small statue.
Return to Merrick. He will now give you your drum. Now you must find another crew to work with, until you are in a group of 5 or above. Use your speaking trumpet for this.
Use your drum near him to copy his tune. You must hold this tune, have one person playing it at all times.
Travel to T26 on the map. Now have everyone play the tune. The beast will rise…. AND YOU MUST SMITE IT!
The Hungering One is extremely formidable, and it is best dealt with using multiple ships. It will easily sink sloops, and in one bite will make an anchored galleon rotate 180°. It will switch to the ship hitting it , and will not chase pirates, but you can still be in the area that it bites. The biggest direct threat to players is the vast amount of sharks which come to assist it.
It is best dealt with by having 3 ships in a triangle formation, with some people in the water with blunderbusses, shooting when it is close, and the people on top repairing and firing cannonballs when it comes out of the water. The pirates may have to jump between ships to repair each other often, so it is advised to stock up on supplies.
Once the beast has been killed, you will be rewarded with the figurehead and shark bite scar. Return to Merrick to finish the adventure, bring justice to his crew and receive your title of Shark Slayer.