Once Merryn is back in the sub, start working your way left.
Go past the smashing pillars and past the statue you reassembled.
Just keep heading left and then travel up to the Tyne Wellspring where you picked up the very first statue torso.
There's an intriguing looking puzzle above you involving mirrors, but you need to upgrade your abilities before you can tackle it.
Take the path up and blow up the wall to your left and take the path that opens.
Blow open the hall on the left and then shove the lantern jellies out of the way with your searchlight and boost your way past the smashing pillars.
At the end, you'll find a red lever that will open a shortcut through the Merrow Ruins.
Make your way all the way back to the right to the branch up shortly before where you started. Start making your way up. Keep an eye out for spawning enemies and coins along the walls.
As you reach the top of this chimney, stick to the left. There is a hidden treasure here worth 75 coins.
Before you continue up, take the path that is slightly above you and to the right.
At the end of this hallway, you'll face (pun intended) another guardian.
Use the same tactics of grabbing his bombs and tossing them back in his face. This one is a little tougher than the first you faced, but you've upgraded a bunch, so it's still a fair fight.
After you've destroyed the guardian, enter the chamber behind him to pick up the Energized Torpedo upgrade. This one lets you supercharge your torpedo shots.
Head back to the left to pick up the main path again and take it up into the Undying Caves.
Boost past the pillars that try to smash your sub and follow the path around to the left to stop at the next Tyne Tyne Wellspring, hermit crab and Tyne Vortex.
Head to the left, past the Tyne Vortex and take the bottom path.
Keep moving left and take out the windmill squid. Head to the top and exit the sub as Merryn. Head to the right to pick up the Tyne Collector, allowing your sub to store more Tyne energy.
Go back to the right and take the tunnel up this time.
Watch out for the Nautiloceros. This room is much smaller than the last time you fought one, though, so it's easier to maneuver around it while it's stunned and destroy it.
Head up to the chest in the top left. Use your claw to pluck the crab off of the top and pull the chest open for a silver treasure worth 50 coins.
Right below the chest is the torso to the statue down next to the Tyne Wellspring. Take it down and place it.
The head is up above you along a Merryn-sized tunnel blocked by currents. Exit the sub and head to the tunnel on the right. Wait for a lull in the current and dash up and to the right. There's a small alcove you can hide in to wait out the next burst of current.
When the current subsides, move to the left and pick up the statue's head. Let the current push you down and take the head to the statue to open the gate on your right.
Go through the gate and let the current carry you to the next room.
Handle the enemies that spawn here, then head straight across to the right and exit the sub once again.
Work your way down past the currents to the bottom of the room where this is no current. Then start working your way back up on the right. Your goal is to move to the right, but there are currents everywhere. Just keep looking for protected areas behind stones where you can catch your breath and plot your course.
Dash to the right, up and into the safe area above the first block. Keep heading up in steps.
At the very top, there is a gold coin at the entrance to a hidden passage that will let you go all the way to the right.
Dive through the current and keep swimming down. Let the current carry you down and head right to pick up the Coral Knife, giving Merryn offensive capabilities.
You can now use the knife to clear the barnacles out of your way. Head left and then take the path up, clearing barnacles as you go.
Keep heading up and follow the path around to the left and then right. Keep heading right until you see the next Tyne Wellspring.
Up and to the left, use your knife to destroy the barnacles blocking the light. This will activate the sensor and open the gate on your left.
Head to the left. At the top, enter the room on the right to retrieve a statue's torso. You'll have to attack the exploding urchins with your only knife, but they die after one hit.
Drag the torso out and to the left to install it.
Head back down and all the way to the left to find the statue's head.
Bring the head back up and place it on the torso to open the gate to your left. Swim out and down to get back to your sub.
Take the sub up and right to pass through the gate you just opened and return to the Tyne Wellspring inside. Blow up the wall to your right to enter the room with the final Resonance Orb.
There's a wall to the right to blow up that will give you access to a chest and a gem worth 100 coins.
Head back to the center and pick up the final Resonance Orb.