Deep Trouble: Crimson Narrows

Make your way to Crimson Narrows by following the river east, north, then west from Chainscrape.

Talk to the injured man, Korvend.

Enter the Crimson Narrows mine, turn left, crouch under a low wall, and dive into the water. Swim forward and surface.

After Aloy calls out, loot the Moderate Supply Cache on a ledge above the water.

Swim to the western end of the chamber and take either the northwest or west submerged passage. The western passage has a side-tunnel to the south blocked until the Main quest 'Death's Door' is completed, leading to a Generous Valuables Cache.

Swim down either flooded passage (use debris to boost by pressing Triangle when prompted) and surface at a platform.

Talk to the miners, Lokuf and Thorden.

Dive into the water to the northwest and swim up the tunnel. Surface, climb a ladder, and continue north to a large water-filled chamber.

Swim to a structure to the northwest, climb out near a ladder, climb a second ladder, and leap across beams.

Duck into a narrow passage that opens into a chamber with three Burrowers. Drop into the water and defeat them.

Make your way to the southern end of the chamber, follow a ramp up and left, then follow tracks until they end.

Jump a gap to the north, turn right (east), and climb a cliff face to reach narrow passages. Loot caches as you go.

Drop down two ledges to the west and make your way northwest to overlook the large submerged chamber again.

Kick a ladder to make your way back up quicker. Use your Focus, locate a Wooden Crane, and drag it towards you with the Pullcaster (aim with L2, switch to Pullcaster with Triangle, fire with R2).

Leap onto the crane arm and use it to reach a higher platform. Follow the tracks to their end and interact with the minecart to drain the mine.

After the mine drains, defeat the Tracker Burrower and two Scroungers that appear.

Reposition a crate under the ladder to the north to reach a platform.

Walk across a rope to the southeast (be precise as Aloy will not cling to it). Loot a cache on the far platform and another on a small ledge reached by jumping on scaffolding.

Drop to the bottom of the drained chamber, exit via the hole in the wall to the south, and climb up to Lokuf and Thorden.

Report to Korvend to complete the quest.