Hey it's Lily42876 here!
Doing my first post! I am going to show you how to get in the hidden lake!
first go to the mine and save the black puffle on puffle rescue save the black puffle and then wait or the shadow to come then keep going. Collect bubbles so you do not sink. Pretty soon, you'll find a maze of coral reefs. Then, you'll find a secret enterence, and you'll be in the hidden lake! get the moss key pin in the secret enterence. That key will get you to be able to go in all the time!