Before you exit the room where you picked up the third Resonance Orb, head to the top left corner. Right below the stone sticking out is a hidden passage that will let you get a silver treasure worth 75 coins.
Head back down and exit to the left, then up and left, back out the way you came in.
Opposite the gate is a light-and-mirror puzzle. The Coral Knife you picked up on your way to finding the third Resonance Orb will let you solve it.
Before you move on, head all the way down and to the right. Exit the sub and use your knife to clear a path to a chest containing a silver treasure worth 75 coins.
Get back in the sub and head back to the top of this room. Exit to the left through the gate you opened earlier.
The on-screen tip is not super helpful here because it leaves out an important piece of information: you have to exit the sub to do what it says. Once outside of the sub, Merryn can use her knife to redirect the light beam into the sensor and open the gate.
Pass through the gate and hit the Tyne Wellspring.
Keep heading left until you see another light-and-mirror puzzle room.
The trick to this puzzle is realizing there are mirrors on the floor and ceiling to help you. Rotate the mirror then use the knife to bounce the light off all three mirrors and into the sensor to open the gate.
Take out the Nautiloceros and start heading down.
You have a choice here. You can use the Tyne Vortex to warp across the map to The Maw, or you can take the long way and collect a lot of treasures and coins as you work your way across and down.
The long way
If you chose the long way, go past the Tyne Wellspring and continue down. Past the first set of smashing pillars, there's another Tyne Wellspring. Immediately above this well on the right is a Merryn-sized secret tunnel. Follow this all the way around for a handful of gold coins.
The current at the end will deposit you directly below where you started. Get back to the sub and continue your journey down to the Merrow Ruins.
Head straight down and take the right to return to that intriguing puzzle room from earlier.
Use Merryn's knife to arrange the mirrors like the image below. Then swim to the corner and reflect the light with your knife to open up the next room above.
Before you head up, realign the central mirror to direct the light upward.
Again, use Merryn to arrange the mirrors to the left. The trick this time is to set the double-sided mirror to let the light pass through when Merryn reflects it.
This will open up the final room, where you can open a chest for a 100 coin gem.
Head back to the sub and continue down. Grab the statue head on your way past and take the left fork down. Use the statue head to open the gate. Before you move on, hop out of the sub and use Merryn to enter the area below and to the right of the statue to find a couple treasures that will net you 125 coins.
Keep going left and watch out for enemies on your way. Stop between the two wooden gates you broke through earlier.
Exit the sub and head down as Merryn and make your way right and down to enter another series of light-and-mirror puzzles.
Open the first gate by repositioning the mirror and reflecting the light with your knife.
Open the second gate by letting the light pass through the double mirror and off of the mirror at the bottom. When you try to reflect the light up into the sensor on the right, you'll be caught in a current, so start reflecting early and let the current carry you to your target.
Reposition the double mirror, then swim right. You'll need to use your knife again to reflect the beam into the sensor.
Set up the mirrors like they are in the image below to open the final gate. You'll use Merryn to reflect the beam right next to the door, then duck through quickly before it closes.
Pick up the Hull Fragment and enter the current at the top of the screen which will carry you back to the beginning of this room where you can head back up to the sub
Keep working your way to the left, past the smashing pillars and take the tunnel up.
Go up past the Tyne Wellspring to the Formori Cyclops. Skirt around it, then exit the sub and squeeze through the tunnel above the Cyclops. Use your knife to clear your path to a chest containing a red gem worth 200 coins.
Carefully make you way back to the sub, then take the tunnel all the way down, through the Easter Island head's mouth and to the very bottom.
Enter the current on the left and let it carry you. When you emerge from the next mouth, take the path down.
Currents will try to carry you around the twisting path, but you can jump out quickly as Merryn to pick up a couple treasures worth 20 coins. Around the first curve, jump out again for another treasure worth 75 more coins.
Continue getting carried by the currents until you see a handle. Pull it all the way down and prepare to rush through the rest of the path.
Keep to the currents and work your way around and through the gate you opened.
On your right, you'll see two statues, a silver treasure and an item. Head to the right to pick up the Dodge Turbine, then swap the head to the other statue so you can make your way back up to the main path.
While riding the current to the left, take the branch up (it's at the top left of the image below) to get out of the current. Use Merryn to explore the path to the left and pick up the treasures and coins. This path that ends in Cara's chamber, but there's nothing to do here at the moment.
Get back to the sub and keep going up as far as you can. Head right and through the next room. Exit to the bottom right. Halfway down the stairs in the following room is a hidden passage to a gem worth 100 coins.
Head all the way back down and reenter the current flowing left where you started this detour. When you reach the mouth of the tunnel, head down and fight the enemies that spawn there.
On the right, there is a coin and a silver treasure to pick up.
Turn around and head up to the Tyne Wellspring and hermit crab. If you want, you can use the Tyne Vortex a little further up to skip to the end of the level and enter The Deeplight.
Use the handle to open the gate behind the Tyne Wellspring and head left into Watcher's Hollow. Just inside the gate is a tunnel you can access with Merryn to collect some coins.
Follow the path left, up, further left and back down. Here, you can get out as Merryn to collect a couple more coins down a hidden passage.
Go back to the sub and continue left. Head down and take the left branch.
Follow the cuttlepup along the hallway to the left just like you did your first time here.
Just like last time, the cuttlepup will drop you off right above a Tyne Wellspring. To the left of the Tyne Wellspring is a stone gate you can destroy to claim a treasure worth 10 coins.
Destroy the next gate down as well. This opens a shortcut to the hermit crab and Tyne Wellsping in the next room.
To the left of the Tyne Wellspring and down is a gold treasure worth 10 coins.
Exit up out of this room and follow the path as it snakes to the left and into The Maw.
You'll come to a Tyne Vortex and a Tyne Wellspring. If you used one of the previous vortices, this is where you'll be rejoining us.
Before heading down, there's a gold treasure in a tunnel directly above the Tyne Vortex.
Head a little further up and let the current carry you further. Head toward the Tyne Wellspring that is on the other side of the hanging anchor.
To the right of the Wellspring and down, boost your way past the current to claim a treasure worth 300 coins.
Let the current push you back up, then yank the anchor off of its chain for treasure worth 10 coins.
Get out of the sub and keep heading up as Merryn. This next chest requires careful timing. Wait for an opening and swim past the Formori Cyclops' eye and behind the hanging stone. If you timed it right, the Cyclops' torpedo will crash into the rock and not you. Go claim your hard-earned prize.
Head back to the sub and then to the left. There are two clockwork cannons along the next tunnel, so just keep moving to avoid getting hit. Blow up the wall and grab the treasure for 10 more coins.
Head over the edge and back down into The Maw.
Keep to the right as you descend. You'll find a barnacle-blocked entrance to an anemone-studded tunnel patrolled by a cuttlepup who's moving like he's had too much caffeine. His speed makes this treasure seem imposing to get to, but you have more time than you think. Just follow him as closely as you can and you'll find a treasure worth 75 coins.
Continue down past the enemies, the new green crab that you fight exactly the same as the red crab, a clockwork cannon, Tyne Tractor Beams and kitchen sinks.
Right below this fight, you'll come to the next Tyne Wellspring and hermit crab.
Head straight left to find the entrance to The Deeplight.