Iron Door

Out of the Muck: Navigate water areas by sailing over blue kelp ramps. You can press A to dash forward.

Clapperclaw Cove: Focus light on crab-like creatures' orange kelp to make it grow. Follow the water to a kelp ramp, drop down, and take a left into a cave past a shipwreck.

Slime Grotto: In the cave, take the right path by breaking a blockade (A). Focus light on Wither sacs on rocks to create paths. Break through blockades with two Wither sacs. Sail left through an archway to an area with blue sacs; focus light on the bottom-left one to remove a Wither blockade. Sail through open rocks. Break another Wither sac to the right, then follow vines to the right to find and break two more Wither sacs to create a path forward. Focus light on a Wither sac on the next doorway, then sail right through a blockade. Break the second Wither sac to your right to open a doorway. Follow the stream forward to reunite with Seabird.

Molter Run: Sail beside two whale-like creatures using only the L-Stick to move back and forth.

The Coral Sanctum: After cutscenes, sail through an archway. Dock at a pink coral house (A) and pick up waiting critters. Behind the house, a gate will fall; send Twig over (X) to lift it. Use the L-Stick to raise it, then the R-Stick simultaneously when baby Twig helps, to lift it fully. Sail through quickly. On the big coral island, deliver three critters picked up from the pink coral house. To get more critters:

Sail right of the big coral island, then right again to break blockades. Focus light on a flower to earn the 'Guardians of the Sea' achievement.

Turn left to find a coral house corrupted by Wither. Find and destroy all Wither sacs to cure it and pick up the critter. Bring it back to the main island.

Sail past the main island to the right corner. Light red/orange corals to draw crabs over, then dock to pick up the critter.

Sail past the main island to the left. Dock to pick up the last critter and bring it back to the main island.

Deliver all four critters to the main island to get a fuse on the right side. Send Twig to collect it (X) and tug it down with the L-Stick. Bring the fuse to the device behind the main island and slot it in (X). Sail toward the powered light and approach large Wither roots to clear the path. Sail over the blue kelp ramp.

The Lock Keeper: Go to the power device in front of the large machine and send Twig over (X) to slot the fuse. To get the next fuse: Sail back to where you entered and go left to a small coral house with lights. Send Twig to the crank (X), use the L-Stick to move the large claw under a water stream. Fill the claw, move it over a crab, focus light on the claw to overturn it, and pour water on the crab to free a floating critter. Go to the locked gold gate left of the house. Focus your light on the eye below the large light to open the gate. Inside the cave, sail right and break a Wither sac to find a fuse. Send Twig to collect it and tug it free. Go back and sail to the left side of the cave. Break Wither sacs to free a trapped critter. Send Twig to the right sac (X) to reveal it. Focus light on the wall to reveal the last sac.