The Hidden Lake was another of the three rooms added to Club Penguin in addition to the Mine Cave and Underwater. Here you can go to the Underwater room (members only). At first you needed the Diving Suit to go to the Underwater room (as shown in the picture below) but then it was changed to a key you get from Puffle Rescue.
But… in the Adventure Party 2011 and in the Celebration of Water was also decorated…