The "Emergency" key?
 

Sometimes I need a key that simply can't be damaged by water immersion. So I bury the remote key fob (keyless entry) and just take the plastic "emergency key" to lock and unlock the car. Which, apparently, also does something non-standard so the car chirps and protests, but I haven't looked into that, since using the fob again cleared that up.

But as I reread the manual, apparently every key is an "emergency" key. Including the metal ones that are in the fobs. So...

If I go to any locksmith and make an extra metal key, or just take the metal keyout of my fob (which will also render the fob safely useless while it is stored that way), that will accomplish the same goal, yes?

Using any metal key, removed from a fob, will give me a "waterproof" key, and inactivate the fob so the car can't be started as well, yes?

Or am I missing something here? (The idea of key #3 being plastic threw me off for quite a while. Apparently that just makes it useless in a fob.(?)

2014 Q5, keyless entry, keyless start.

The OEM FOB will start the car without the key attached. I start my car in the winter to warm it up, pull the key from the FOB and manually lock the door with the engine running while I go back in the house. Yes, someone can smash the window, unlock the car and drive off, my area is sort of safe, but some kid could get in and drive away.

That OEM plastic emergency key has a chip in it to allow starting the car. Getting a locksmith to make you a new metal key will only allow you to unlock the doors and not start the car. You need the FOB chips to start the car.

Bob is right. To provide a little bit more detail, you can start the car with the emergency key only when used with the little fob adapter in the glove box. No need for the full OEM FOB with the metal key.
If you want a waterproof solution, leave the OEM FOB at home, take the emergency key with you (to close and open the door), and once in the car, insert the emergency key into the large FOB-like plastic adapter that is in the glove box to start the car.
By the way, as you have the keyless entry, I believe you can just place the emergency key on the dash (I don t have the system but there should be a icon or something to locate the position) and then start the car by pressing the start button. The car will read the ship inside the plastic key but the key needs to be very close to that location.

Not quite sure from OP's post what the objective is. Other relies have already described the chip element and removable key portion. But if it is truly just to get the door open or locked without any starting, then you can buy an older Audi key blank with a conventional plastic head and metal blade area. On EBay if not elsewhere. Those are what used to be the form of the main "valet" type key. Then you go to a locksmith that makes sidecut keys and get it cut. I did that with my 2000 A6 4.2 that was just a year before chip keys. There it also started the car, but certainly opened and locked it too.

So there are two ways to skin the cat.
The original objective being, as stated, the need to have a water submersible key for the car.

Either I can use the plastic key, in which case it will do everything if I also use the plastic key holder.

Or I can use any metal key from my fobs, including a copy made by any locksmith, in which case I just need to keep the fob instead of the plastic key holder in the car.

In the later case, someone could steal the car simply by using the fob, but with the plastic key, they can't start the car without the actual key, the holder is just there to make it fit in the dashboard properly, the actual chip is sealed in the plastic key. Which would only be available from Audi.

Yes?

Sounds like using the plastic key is the most effective and safest way to go.