From the room where you fought the Watcher, continue following the path right, using your new torpedoes to blow open any gates in your way.
Keep heading right until you see a wellspring and the hermit crab.
Head up and to the right and back into the area where you first picked up the boost ability. There are a lot of new, tougher enemies around the next wellspring on the right.
Once you've defeated them, boost through the current on the right and use your torpedoes to open the path.
Clear away the lantern jellies with your searchlights, and you'll be faced with a couple waves of the new, harder enemies. Boost into the tunnel on the bottom right if you need some breathing room, but don't think you can just run away. They will follow you.
Once you've cleared all the anglerfish and nettles out of the room, head to the top left. Torpedo open the path, and keep heading up and left.
You'll be faced with a few of the new, yellowish anglerfish, but picking this fight is worth it because you'll receive the Super Charged Claw.
Head back down to the bottom right.
There's a handle that opens the gate up ahead, but you've got a very strong current to deal with. Pull the handle and boost forward to the alcove during a lull in the current. Wait out one of the current's cycles in the alcove, and then you can boost your way up to the gate and through.
Hit the red lever to keep the gate open, and then you can head back down to grab the silver treasure.
Just to the left of the gate is a wellspring. Break through the gate in front of you, then head left.
You'll face a few more enemies. Head to the bottom right once they're defeated. There's a chest that will give you a treasure worth 75 coins.
Head left and break through another gate. Keep heading left, and you'll find a deceptively simple puzzle.
Head along the middle path and pull the handle. Use the current along the top path for a speed boost to travel back to the right. Drop down to the bottom path to boost through the gate. Hit the red lever to open the gates and pick up the hull fragment.
Head up and left to return to the wellspring and crab again.
Take the path down behind the crab, and then head right into the Merrow Ruins. There's a red lightning crab here to greet you this time. Take care of it and the various other enemies, and then hit the wellspring for a quick save.
Head straight down and, once again, use your searchlights to clear a path through the lantern jellies. Keep going past the blue clam you fed earlier.
Turn right and use your torpedoes to open up your path forward. Pull open the chest for a gold treasure.
Work your way back across the roofs and all the way to the left. Head down past the Tyne Vortex to the wellspring and crab.
Since the enemies have gotten so much tougher, it's time to upgrade to the Alloy Claw.
Head down between the wellspring and the crab, and take the first right. You'll be making your way forward against the current using the boost-and-claw anchor method you've used before. Take the path up when you see it, and torpedo your way into the next room.
Head up and to the left to hit a wellspring.
Move over to the right, and swap the head on the statues. Go through the new hallway that opens as a result.
Kill the enemies that spawn from the ceiling, then head to the bottom left. Pull the handle and quickly grab the statue head.
You don't have time to make it to the gate, so just boost-toss the head through the gate before it closes.
This will open the hallway at the bottom right. Pass through it and head straight up until you see a wellspring on your left. Save and then torpedo through the two walls to get to the chest.
Get yourself another hull upgrade. Geography note: This is immediately behind the chest near the blue clam.
Turn around and keep going up to encounter a(nother) Formori Cyclops Turret.
Sneak past, and head to the left and up. Pull the handle, and pass through the gate. Don't worry about the statue's head yet.
Pick up the Diving Suit and breathing shell, which will allow Merryn to exit the sub.
Use Merryn to drag the statue's head down and left to hold the gate open. Either swim back to the sub or hold the button to return automatically.
Take the sub out to the left and down. Hop out of the sub again, and collect some coins.
Keep heading right and park the sub in the Cyclops' blind spot, then hop out and grab the silver trophy.
Sneak back past the Cyclops and keep heading down. Go through the Easter Island head's mouth, and torpedo the precarious rock from the ceiling. It will fall on the chest below it to get you a treasure.
Head left and let the current carry you all the way back to right under the wellspring and crab.
There are more enemies you'll have to deal with on your way up to the save point. Head back down, and work your way back through the current on the right until you get to the path up. Take it again, but this time, stop halfway up and let Merryn out.
Following the winding path around.
When you see the branch leading up to Cara, keep heading left. Work your way down to grab a gold treasure, but be prepared to run away because Merryn's has no offensive abilities (yet).
Head up and have a conversation with Cara about the Deeplight.
After they finish talking, two gates open on either side of the room. Return to the sub, then take it up and left to return to the wellspring.