To find the key is pretty easy. You will have to play puffle rescue with the black puffle.
It's on the very first level.
When you get the black puffle, don't go back to the finish line. You will have to wait for the squid.
Follow the squid using the bubbles for air. Make sure not to shrink and die.
Follow the squid until you find this area. There you will find the secret entrance.
This should pop up when you find it. If this doesn't pop up, make sure to clear cache and try again.
Following these instructions should grant you access to this beautiful hidden lake. The key to the hidden lake can also be used as a pin on your player card.
Hope you enjoyed!
Waddle on.