This is how to get past the door in the Hidden Lake:
1.Go to the Mine.
2.Play Puffle rescue, choose the black one in the ocean.
3.On the first level go to the Puffle and DON'T come back, wait until you see a shadow of a squid under you.
4.The squid will blow bubbles that you can jump on, jump om them while going forward.
5.Continue until you get to a small island.
6.When you get there turn right and go through the wall, there you'll see a staircase.
7.Go down and you will find yourself in the other side of the door in the Hidden Lake!
8.The key to the door is inside the room so you can grab it and return whenever you'd like!
If it doesn't all make sense there's a tutorial video: