1 Solve the control room puzzle
To get the Gallery key, you need to drain the water from the areas filled by water. Go to the control room at the circular area in the bottom of the Aqua ring and solve the control room puzzle.
2 Get into the round corridor
Get into the round corridor where the body of the large shark Neptune lies. This area is at the bottom of the Aqua ring.
3 Go to the end of the corridor
Go to the end of the corridor, get into the water then climb up the small platform. Don't worry, Neptune won't attack you.
4 Push the control unit in the water
Next, you need to push the large control unit down into the water. This step is needed to kill Neptune as it will wake up again.
5 Push the switch
Push the switch next to the control unit you pushed in the water. Electric current will now flow into the water and Neptune will die.
6 Grab the Gallery key
Search for a shiny object right next to Neptune's body. That's the Gallery key.