Getting the Blue Key / Getting to the Blue Key Slot (4)

You'll start the level with two Large Medikits right behind you. Look around, and you'll spot a crack in the wall. The thing you need to interact with is actually a dumpster right in front of that crack – blast it open! Inside, you'll find a Pistol and some Chaingun Cannon Ammo.

Keep an eye out, there's a Pig Cop chilling on a ledge nearby. Use the side of that dumpster you just opened to hop up there. Once you're up, you'll find Shotgun Shells and another Large Medikit. Just as you grab those, expect to be attacked by a Trooper, a Captain, and a Pig Cop across the way.

If you took some damage and need a quick fix, there are two Small Medikits on the other side of the river.

Now, follow the waterway downstream. You'll find some Pipebombs on a ledge. Here's a pro-tip: you can go underwater, but before you dive in, toss some of those Pipebombs down there. It'll clear out the Octabrains lurking below. Once it's safe, dive in and grab the Shotgun that's on a shelf, and then the Shrink Ray lying on the ground.

Emerge from the water where you found the Shrink Ray. You'll come up onto a ledge that has the Red Key door. Right here, you'll find the Blue Key you're looking for, plus a Portable Medikit and a box of Shotgun Shells.

Once you've got all that, you'll need to backtrack to the Entrance area of the level. Use the Blue Key you just picked up there to proceed.