You should see a platform below with a brain slug hanging above it. Below said brain slug is a crate we need to move to the left. Wait until the brain slug is swinging away, then run in and pull the crate to the left. Jump over it and let the brain slug get you.
It will compel you to walk left, do so and use the crate to jump up to the ledge from where we entered. As you hit the light, the slug will force you back to the right. Continue past where we first encountered the brain slug and jump over to the first ladder.
Wait for the falling debris to clear before jumping to the next ladder. Climb down and jump across the platforms to the right until you reach the next light source. You will notice the water is now rising, so we will need to be careful on the way back. Once you return to the first ladder, jump to the second and once again climb up to the previous area using the box on the left side of the platform.
Once the slug compels you back to the right, climb the first ladder, climb to the second and then wait at the top. A loose piece of piping will float to the surface. When the water level is high enough you need to jump to the pipe and then from there across to the right-hand platform. Let the demonic growth on the ceiling deal with the brain slug for Let the demonic growth on the ceiling deal with the brain slug for you. Climb the ladder for a moment until the water stops rising.
Prepare a dry landing
Return to the ladder and climb to the top.