(left side)

First up, you're in the Maintenance Room Passage. Keep an eye out, there are two Ghiozzo enemies in the water here. Just whip out your machine gun and take them down when they start moving towards Jill.

Next, you'll enter the Engine Room Corridor. You'll see an Ooze enemy in the water under each of the gate doors leading back to the Engine Room. Your goal is to open the north gate to get back to the Engine Room (Upper Level). Parker will chime in with some chatter. Just run past him and open the door on the other side of the room.

Pro-Tip: To make getting back to that gear puzzle later a breeze, there's a shortcut. Open the door on the left side of the Engine Room Corridor, then backtrack through the lower level of the engine. Head up the stairs through the next door, and Jill will be right in front of the gear puzzle. This way, you won't have to fight any enemies on your return trip! I'll explain the route with enemies below, but seriously, take this shortcut if you can.

Now, you're in the Water Corridor. Turn right and step down into the water. Be ready, there's a Sea Creeper in the water to your right. If you have a shock grenade, toss it in as soon as it appears – it'll take it out instantly. Keep moving down the corridor. Another Sea Creeper and an Ooze will pop up from the water. Looks like another job for a shock grenade! Toss one in near them. If Jill's out of shock grenades, blast the Ooze first, then just run past the Sea Creeper.

Head up the stairs at the end of the corridor. You'll be back near the set of gears. Place the cog you've got onto the middle part of the gears, then hit the button next to the gear wheel. The middle section will open up, revealing the RESTART KEY. Parker will radio you once you grab it – repairs are complete! To avoid fighting any enemies, just head back the way you came and return to Parker in the Engine Room (Upper Level).

You're now in the Engine Room (Upper Level). Your objective here is to restart the power. Parker will be waiting and will ask you to go to the other set of controls. Hurry over to the controls on the south side of the room and use the restart key you just found to restore power. The water level in the room will start to rise, and a cutscene will play out shortly.