The direct path to the Forbidden City is still blocked to you. Once again you have the choice to warp to Merrow Ruins right above the entrance or take the long way around. Watcher's Hollow is pretty well cleared of useful items and easy-to-get-to treasures, but you haven't been back to Skeleton Reef in a while. From the Tyne Wellspring outside of The Deeplight, go up all the way to Skeleton Reef.
Head right and through the enemies that spawn around you. Continue right then up and follow the path as it snakes around to the mines.
Keep heading right until you can drop down.
To the left is an Easter Island head. Use a magma bomb to plant a grab point on the head and pull it out of the way. Duck behind it to pick up another Tyne Collector.
Push past the stone head and to the right. Exit as Merryn to collect a gold treasure.
Take the sub up and enter the ship above you. Go through and to the right. Pull the handle on the bottom path, then rush up and through on the top.
Take Merryn through the small tunnel on the top right and claim the Hull Fragment.
Go back to the sub and exit along the bottom path.
Take the sub up, left, and up again, almost to the top of the map.
There is a tunnel on the left with a stone head inside. Repeat the magma bomb trick to move it out of the way and collect a silver treasure for 50 coins.
Outside the tunnel, down and to the right is a Tyne Wellspring and hermit crab. If you have the coins (and you should after all the treasure hunting), pick up the Helix Torpedo upgrade to fire "two torpedoes for the price of one."
Keep heading right and go down into the shipwreck through the opening in front of you.
Continue to work your way down through the ship. Past the next Tyne Wellspring, you'll have to fight your way through some enemies. Keep going down and a little to the right until you see two skeletons on either side of a tunnel down.
Head down until the tunnel gets too narrow and exit the sub. Follow the path that continues down to pick up three treasures worth 10 coins each, then head to the left to keep following the path down.
Below where you picked up the treasures, you can pick up Seahorse Parts.
On your way back, stop to pick up the glowing rock. You may notice that some of the fish that are usually in the background are following you. Lead them over to the silver plant surrounding the chest and they'll eat it for you..
Open the chest for a treasure worth 300 coins, then return to your sub.
Head straight right until you see a gate with a handle. Pass through it and continue right until you can drop down into Watcher's Hollow.
To the right, there's a strong current pushing down. With careful use of your boost, you can make it up to the chest above and claim a Hull Fragment.
Let the current push you down and head as far as you can to the left. There's a stone gate above you that you can destroy in one shot now (if you bought the Helix Torpedo upgrade). Open the chest for a gold treasure.
Head straight down as far as you can, then head to the right.
Pass your old buddy The Watcher again and continue on until you see the sea snakes along the floor. Boost past them from left to right to scare them all at once and they should reward you with a silver treasure.
Repeat the boost-and-scare with the next set of snakes to the right for another treasure worth 10 coins. Head straight up from the second set of snakes and take the second right ‒ the one after the one with the current. Head to the upper right of the next room and ping your sonar to reveal a hidden passage.
Send Merryn up and through to pick up another Tyne Collector, but watch out for the enemies that spawn in that room.
Head all the way back down and to the right. Drop down the tunnel behind the hermit crab and take the path right into the Merrow Ruins.
Go down through the lantern jellies again, then head to the left and down.
This Tyne Vortex is where you would have warped to from The Maw.
Continue heading down. There's a large stone head on the right facing forward. Ping your sonar next to its ear to receive a red gem.
Keep going as far down as you can go, then head right to find a sonic lock.
Pass through the gate and completely ignore the warning of the clockwork seahorse.
You'll be faced with a pair of Formori Sentinels. Try to keep moving to avoid their torpedoes and make use of your own upgraded torpedoes. If you need to catch your breath, boost to the left to try to make some room.
Once you've defeated them, continue right into the Forbidden City.