How to Get The Underwater Key
The Underwater Key is for members, if they want to access the Underwater Room. To start off, go to the Mine and play Puffle Rescue.
Do the Black Puffle levels.
On the first level, get the Black Puffle.
Stay on that tile and a few seconds later an octopus will appear.
Follow the Octopus, using the air bubbles. You will eventually reach a secret entrance. Go on top of it.
You will end up in the underwater room. Get the key.
Now that you have the key, you can access the underwater room from the Hidden Lake.