You start this level with a big box in front of you, push it on the other side, kill the rats around and position the box so you can reach the switch.
Jump into the hole that just opened and drop down to the lower level.
And you'll soon reach this huge room. It will take some time to get used to it and find out what you need to do, but first I suggest you try to do something about the crocodiles in the water. In order to do this though, you need to get rid of the water.
You can try to shoot them from the edge of the water, but from my experience didn't work too well.
So here's what you should do. First, jump into the water and swim out before you take damage from those sharp aligator teeth. They are slow so with a bit of help from the E key you should be safe.
You should find a wheel nearby, use it to get rid of the water.
And then it's target practice with those buggers.
Notice there are 2 pads, so you will need two crates to push those.
Here's how to get out of there too, you will need it sooner or later anyway.
Find this rock first, pick up the small medikit since you are there as well.
Jump on the slope and slide a bit, then jump again and use the grapple to get across.
It's a bit of work, but it's easy to see the way you need to go.
And in the end you should be back up.