This is a walkthrough for Flood Zone; the 3rd level in Shrapnel City. This is based on Let's Rock.
When you first start out, you are in a rather dark corridor with some light at the far end. Once you move forward, an Octabrain will come after you if move to the waterfall. Also from the start point, if you turn around backwards, there is a compartment in the wall which contains scuba gear which you will definitely need for the underwater sections. Make your way to the ledge at the end of the waterfall and when you do you'll see the top of a building you won't be able to get to yet; assuming you don't have a jetpack as this walkthrough is based on the player not using the jetpack but aside from being able to explore some areas earlier with the jetpack the player will still have to go to the main places to get further in the level. Anyway when you do make your way to the ledge, take a jump down, you will land in water.
Once you land in the water, make your way around the side of the building, and you will see part of a red neon sign and an underwater cave. Swim through the cave which is just a bit beyond the partially submerged section of the red neon sign. A word of caution ; in underwater areas in this level there are lots of sharks. While they don't pose much of a threat to you, they can get in the way of the shots you may try to take at others like Octabrains, so it means if you fire an RPG round at an Octabrain and a shark happened to be too close you would end up killing yourself.
When you make your way into the cave you will see a ledge with an Atomic health on it guarded by an Octabrain. Take out the Octabrain, and you will want to surface from the water in the area right above the Atomic health.
Once you surface, you will find yourself on a the roof of an almost completely submerged building. At the back of the roof is path leading upwards on the rocks. You need to make your way over to the bottom of the ledge, and start hopping up the path.
As you make your way up the path, you will encounter a ledge with some Devastator ammo on it, and the blue access card. Take all of this. From here you see a darkened area which is just past the blue access card. You can jump over to the darkened area if you like and go further up; if you do go upwards to the Red Access Card door which is on top of the building with the red neon sign. Going up there will release several enemies on top of the building on the other side of the level which you will have to take out; these enemies include a Mini Battlelord and an assault commander and you'll also see some enforcers on top of the other building. You don't have to go up here yet if you don't want to but it's good if you want to clear the area before going to this door later on when you do get the Red Access Card. Either way you'll have to come here later.
Once you are done fighting with the mini battlelord or you have decided not to go up to the Red Access Card door yet jump into the water below. You will want to make your way to the door on the smaller submerged building you were on top of previously; except you'll be underwater. This is where you use the blue access card.
Once inside, there will be quite a few Octabrains in here as well as those annoying protozoid slimers so be careful with explosives. The room is quite small; so be prepared for a lot of close quarters combat. Once you are past all of that, the yellow access card is in the back side of the room. Go back there, and make your way out of this building.
After you are out of the yellow access card building, make your way back to the initial part of the level when you first dropped off the waterfall. While still underwater, you will see a window that leads into the building; you have probably seen this window already when you first jumped in the water anyway go in there, and use the yellow access card. If you surface in this room, there is a medkit over in the corner which can be useful if you are low on health.
Go through the yellow door access card door and start to make your way down the stairwell or if you like go up to explore the upper section of the building first; some of the upper section will already be quite familiar to you as you would have seen some of it when you first jumped into the water at the start but wouldn't have been able to explore it without a jetpack and when you get up there you'll find quite a few enemies; as well as that when you reach the area opposite the building where the red access card goes a new wall near the red access card door will open up revealing a couple of assault commanders for you to deal with. Anyway wheter you explore the upper section or not you need to make your way down; most of the stairwell is all underwater, and very dark, so if you have the night vision goggles, use them, it will help you out. On the way down the stairwell, you will run into an Octabrain. Additionally when you reach the bottom, there is a lift across the hallway with another Octabrain that will attack you. Be careful, as explosives in this area will usually result in you taking heavy damage. The shrinker is a good choice here if you have it.
After killing the Octabrain at the lift; turn right at the bottom of the stairs you will come out into an office room with several Octabrains you need to take out before you progress. After you've done that, you will see a switch near the closed windows. Press it, and the windows will open, revealing the previously visited outer area. However, the red neon sign on the far wall will have risen, revealing the red access card. Take it and you will need to head back up to the top of the level where the locked red access card door was that you might have encountered earlier; if you haven't been up there already you'll have to deal with the mini battlelord that you had the option to take out earlier on; if he's already dead it'll be plain sailing to get through the red access card door.
Inside the red access card door is a bunch of crates with another section of rock face behind them as well as a few enforcers. When you kill them make you way over to the hole behind the rock face. Take out any Octabrains you encounter and advance further down the passageway. You'll come to a large drop and on the other side of the large drop is atomic health which you might find useful. Anyway Jump down and you'll end up underwater; when you surface be prepared for one last nasty surprise. A turret is on the floor which is a small platform opposite the exit and it will shoot you in the back while you try to go for the exit. Destroy the turret and then hit the exit as it could take a good bit of your health otherwise. Anyway when you hit the exit it's onto L.A. Rumble.