What you do is to follow the shadow RIGHT behind and let his bubbles pop on you to keep you alive. I hope this answer is helpful!
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On the black puffle rescue get the puffle and wait there until you see a squid shadow and then follow it but keep walking into the bubbles so you don't sink and then the shadow will lead you into a secret place and find the key! that's all
On the first level, get to the black puffle and then follow the shadow of the giant squid. Use the bubbles to float back up. Once you are at the end, follow the maze to the stairs.
In Puffle Rescue there is this secret octopus shadow in the Black Puffle levels. Keep following it and it will bring you to a secret level, and if you beat it you can get to the secret underwater room and you get a pin that looks like Rockhopper's Key with seaweed on it!
Play the black puffle rescue and follow the Shadow of the octopus and then you are in the secret room and you have the key.
you just go on the bubble but first you go to the puffle rescue game and you have to be a member OK then you go straight on the black puffle one when you get the puffle wait for the Shadow to come then when the shadow passes you go on the bubbles intill you get to land when you get to land go to the right and go intill you find a door go in that then the key is there