Where is the red key card?

The red card will open the door beyond where the blue light is, near the
exit.

First off, it's underwater.

To get it, you'll need to deactivate the force field surrounding the tunnel
that'll get you underwater.

Get into the out-of-order elevator by blowing up the barrels next to it.

Hit the switch inside the elevator.

Now, go up to the door that's lit with the blue light (the area itself is
green).

There should be four lights across from the door.

Deactivate the one light that's glowing, and activate the second from the
left.

It should look like: OFF OFF ON OFF.

This will lower the force field back outside.

Jump into that area that used to be cut off, and dive underwater.