After you complete The Eye of the Earth main quest in Horizon Forbidden West, you get three main quests to complete in whichever order you want. One of these is The Sea of Sands, in which you have to search for the POSEIDON subordinate function. In the first part of the quest, you go to the ruins of Las Vegas and, in collaboration with a group of delvers, craft the diving mask Special gear so that you can breathe underwater. But once you have the diving mask, how do you find POSEIDON?
Dive into Dunehollow, the flooded area in the Tower of Tears (where you retrieved the compressed air capsule), swim to the bottom, and go through the gap in the east wall. Follow the marker deeper, and further east. In the underwater dome, go to the northwest side, avoiding the Tideripper on the way, and when you get close to the large hole in the floor, your objective will change to “Find the Red Light.”
Find the red light in a cave area south of the hole, and activate the console. Follow the marker to the primary pump node access point, then swim through the pipes to find the node itself. Swim under the gate on the north side of the chamber, and climb out using the ladder. Open the valve, then swim back the way you came, and follow the marker to the second access point.
Again, you need to swim under a gate to reach the ladder, and you’ll need to jump to the top of a gate to reach the valve. Swim back the way you came, and follow the marker to the Pump Maintenance Station. Get in via a hole in the ground on the east side of the Pump Maintenance Station. Pry open the door, then head west, then north, and open another door. Swim up to the console and activate it, and the water will drain from the dome.
Climb down the ladder, then swim back the way you came. Climb out of the water hole and follow the marker north. Fight or avoid the smaller machines. When you’re close enough to the marker, the delvers will show up. You’re going to have to kill the Tideripper. It is vulnerable to ice and electricity, so make use of whatever traps and weapons you have of those two types. It also has a lot of components to target although, as you’d expect, the most critical ones are the hardest to hit.
Have a brief chat with the delvers if you want to, then swim across the lake and open the door. Turn right, follow the corridor, and go down the stairs, then go through another door and down more stairs. Use the console on the other side of the room to restore the POSEIDON subordinate function to GAIA, then retrace your steps and climb the ladder hanging off the broken bridge. Follow the marker into the hotel/casino and turn left at the top of the escalators. Go up another escalator, then go around to the right and through a hole in the wall. Go north, then through the doorway on the right, and climb the ladder. Continue west then use the elevator to get back to the surface. Interact with the curtain to talk to the delvers again.
To complete The Sea of Sands, and get the Recovered POSEIDON trophy, return to the Base and deliver POSEIDON to GAIA.