Go down and walk straight. You will get to another room. Examine the left wall. There's a hole. Walk through it.
You will get to a door behind which there will be a zombie. Be ready for a quick retreat. The zombie will be strong enough to smash not only the door but also the wall.
When you kill the zombie, walk through the smashed door. You will get to a room with a hole in the middle of it. Remove the planks covering the hole and jump in.
Go straight. You will get to a lift. Open the hatch and step inside.
Kill the zombies and keep going. You will get to the stairs. Go down.
You will get to a door. Walk through it. The next room will be full of barnacles. Give them a barrel. Use the gravity gun to do it (LMB). When the barrel is up you are free to go.
In the next room a passage will be blocked. Remove the planks.
Afterwards, clear the ceiling.
When it's safe to go, you will have to run to get to the other side. The planks are rotten. Press the sprint key to run.
When you're there, turn off the valve. It will put out the fire.
Go straight and down the stairs. You will see a zombie banging at the door. Kill it.
Behind that door, there are some goods. As soon as you walk in, the light will go out and zombies will surround you. If you like fighting, choose that option. If not, just open the door and get the goods out with the gravity gun. Then go to the stairs behind you, and then turn right. When looking at the previous screenshot, the stairs mentioned above are to the right.
You will go outside. A zombie will attack you. Kill it.
Then get to the fence on the right. You will see that there's a padlock.
Remove the padlock with a crowbar, for instance. You will get to a narrow passage full of zombies. Kill them. Alyx will help you.