From the Skeleton Reef vortex, head up and to the right.
Fight your way through the enemies. You're looking for the second tunnel down.
Blast through the magma gate and use Merryn to grab the green gem above you for 100 coins.
Squeeze the sub through the gap to your left to get a green gem from the anemone.
Use the provided mine to blast your way through the metal wall above you and to your right. Fight through the enemies and grab the two gems worth 100 coins each.
Head back down and grab another mine to use on the metal wall to your right. The best solution we found for getting the mine to touch the wall enough to destroy it was to simply brute force it with the sub. You'll take some damage, but you've picked up enough Hull Fragments to make it negligible.
Take the path up at the branch ahead.
Fight through the enemies and continue up and to the right.
As you continue on your way up, watch for a stone head. Hover on the right side of the head and ping your sonar to get it to spit out a red gem.
Press on against the currents and keep heading right.
Head right until you see a gap in the glowkelp near where you very first started your adventure.
Blast through the stone wall and head down into the round room with the current. The goal is to use one of the bombs to blow up the metal wall to the left. The best way we found to do this is shown in the video below. Pick up a mine and let the current carry you past the wall, then turn and boost throw the mine behind and above you. With the right timing, this should take out the wall.
Duck into the hollow in the rock in the center to get a gold treasure. Let the current carry you around one more time, then exit to the Tyne Wellspring.
Use your searchlights to lure a school of those tiny fish along with you as you head down into the next room.
Bring them to the silver plant on the right to give you access to a chest and a Tyne Collector.
Go to the right and blow up the glass gate. Keep heading right to find a guardian protecting the final Clockwork Seahorse Parts.
Head to the bottom left of this room. If the guardian's bombs haven't already blown up the metal wall, grab a bomb from the room to the left. Collect your 200 coin gem.
Head back and all the way to the left to find a gate. Open it and continue down. Ping your sonar to find a secret passage
Continue down to a well-guarded area of the Undying Caves and a chest containing a silver treasure.
Continue down and to the right to the Tyne Wellspring, hermit crab shop and Tyne Vortex. Take the Vortex to the Merrow Ruins.