Head immediately down and to the left where you'll find a hollow under a roof. It's blocked by some pottery, so unleash your inner underwater Link and smash through them to grab the coins.
Head back up and to the other side of the roof's peak to the next save point.
To the right of the wellspring, the next Chekhov's Gun blocks your path and provides a hint at your goal to this area.
Head up and to the right, continuing over the ruined rooftops. After the third peak, you'll find a clockwork cannon that doesn't seem too happy to see you.
The torpedos it fires don't track you, so just keep moving and use your new boost ability as necessary to move past it.
Head down until you see a path to the right. Take this to the next wellspring.
Follow the tunnel up, taking care to avoid the Lantern Jellies. Follow the path left and carefully work your way past the first set of pistoning pillars.
Time it right, and you'll be able to just move past them, but feel free to give yourself a boost if you're more comfortable. Continue up and past the second set of smashing pillars.
Continue up to the third level where the pillars shake and move, but don't actually threaten you. Here you will claim the Searchlights.
Use the searchlights to drive the lantern jellies out of your path and make your way back to the wellspring. Continue back to the left where you'll encounter a mysterious new creature.
Go down from here to find a room with more coins, enemies and pottery.
Head back up and to the left, back past the clockwork cannon and across the rooftops toward the first wellspring. You'll bump into the merrow maiden again.
With your searchlights installed, you can now travel down from the wellspring. Shoo the lantern jellies out of your way and keep to the right wall. If you upgraded your claw back in the Undying Caves, you'll be able to break through a pair of wooden gates just past the blue clam to claim a couple treasures worth a few coins each.
Make your way back up, then head down the next tunnel to the left. Carefully target and grab the turquoise brain coral. Take care not to break the pot, though.
Return to the picky blue clam with this to receive a treasure.
Keep heading left until the path opens up below you. Keep heading down and you will come to a Tyne Vortex and another encounter with the merrow, who tells you to call her Cara.
Keep heading down to find another wellspring and your old friend the hermit crab. If you have the coins to upgrade to the Boost Turbine, the extra speed it gives you is worth it.
Head down and take the first branch to the left to find a chest containing a Hull Fragment to increase your max health.
This is effectively the end of the level with your sub's current specs. There are a few coins to find through exploring, but if you're more interested in progressing the story to the next area, head back to gate to the left of the wellspring and skip down to the last paragraph.
Almost directly across from the hull upgrade tunnel is another leading to the right. Careful timing with your booster and grabbing onto the various anchors along the way will let you progress against the current far enough to net a couple gold coins, but you can't make it too far.
Head back to the main tunnel and keep going down. Watch for coins and enemies along the walls as you work your way down.
Once you reach the bottom, the right path also requires abilities you don't yet have. Head left to see the clockwork seahorse waiting for you to progress the story and your improve abilities.
id="last-paragraph" Head back up to the wellspring and crab. Head to the left and use the handles on the chains to open the gate to Watcher's Hollow.