Boiling Cavern is the 7th chapter in Sword of the Sea. This guide contains all collectible locations and everything you need for trophies/achievements.
Collectibles Overview:
- Sword of the Sea Collectible Guide
Boiling Cavern Collectibles
Secret Shell #45 – When you reach the area with the boats floating in the lava, you need to quickly jump around the back of the pillar on the left to find the shell on a small ledge.
Stele #21: The Fires Below – After opening the door with the water bubble, the stele is on the right side of the room just behind the rock.
Secret Shell #46 – In the same room as the stele, jump up the rocks behind it and follow the path clockwise around the room, then jump across to the far ledge where the shell is located at the top.
Stele #22: The Army of Red Fire – In the large room with all the boats, stick to the right side where you can find a stele around the corner.
Secret Shell #47 – On the right side of the area near the stele is a boat sideways in the lava. Jump on it to reach the ledges and follow them to the right where there is a small cave with the shell.
Secret Shell #48 – Near the start of this area to the left is a round platform, if you look past it there is a thin corridor in the rock. Quickly jump through to the back where you can find the shell.
Frog Skeleton – From the platform where the water ball is located, go to the right across the boat and up to the one on the tall platform to find the frog skeleton on top on a lantern, awarding the trophy/achievement Frog Skeleton 
.
Stele #23: When Water Battles Fire – When you reach the room with the skeletons you can grind, the stele is on the second level at the end of the second one. If you interacted with all prior stele you will earn Wisdom of the Ancients .
Secret Shell #49 – Behind the stele, you can jump up the ledge behind a statue to find the shell, awarding Boiling Cavern Shell Collector .
Secret Shell #50 – The final shell comes from the final trade level of the vendor for 5000 Astra. Depending on how thorough you are with Astra collection you might have enough to get this in first playthrough, but if not you can get it in NG+ or Chapter Select. The final chapter “Sky Abyss” doesn’t contain any collectibles. Finishing the story will award the trophy/achievement Loyal Customer 
for receiving the last reward, along with Water Whisperer 
for obtaining all secret shells.
This finishes the collectibles in Sword of the Sea. If you missed any item, you can still find everything through Chapter Select (unlocks after the story). For an overview of Collectibles by Type refer to Sword of the Sea Collectible Guide.
