You need to find the key at the Ship Graveyard
Phlegma: "You'll need to go to the Ship Graveyard by yourself. I'm going back to Berk to have Gobber and Gothi look at my injury before it festers. I'll catch up with a clean bill of health. Go!"
1- Go to the Ship Graveyard
Mulch: "Phlegma told us you need backup! Bucket and I are ready to get our hands dirty. Come over and talk to me!"
2- Talk to Mulch
3- Look for the 'Dragon's Jaws'
Mulch: "I can recognize Dragon's Jaws when I see them! Now, pull out your compass and head north of the sea stack. The shipwreck should be somewhere near here."
4- Search north of the 'Dragon's Jaws'
5- Search east
Mulch: "There! That sea stack! It's large enough to break the flow of the current. Maybe Hamish's ship is down here.
Do you see anything that might give us a clue that Hamish the Second's ship lies far below us?"
6- Look for clues of the shipwreck
Mulch: "Eureka! That looks like the tip of a mast to me. We must be on the right track!
Bucket has the best set of lungs among us, so I'm sure he'll volunteer to dive down and find it! Come by and talk to me and we'll sort it out."
7- Talk to Mulch
Mulch: "Do you see anything, Bucket?"
8- See if Bucket needs help finding the key
Bucket: "It's no good, Mulch. It's really deep, and the air in my bucket makes it really hard to dive deeper. I'll keep trying."
Mulch: "Bucket is as healthy as a yak; if he can't swim to the bottom of the sea, there's not a Viking alive who can! maybe we can tie something heavy to me with some rope. I'll have a few moments before I drown, right?"
Phlegma: "Ahoy there! Sorry for the delay but I've arrived with reinforcements. We're here to help!"
Johann the Trader: "Young dragon trainer, you have a permission to board the ship to talk to the lovely Botanist. What a wonderful - and generous - Viking!"
9- Talk to the Botanist
10- Click on the diving bell
!: "You should swim around by the ocean floor and look for the key. When you are running low on air, you can return to the diving bell and breathe from the air trapped under the bell."
11- Use the bell for air and find the key
!: "This must be what you were looking for! You should return to the surface of the water by clicking on the rope tied to the diving bell."
12- Click on the rope on the bell to return to the surface
Phlegma: "Well done, [your Viking's name]!"