Unlocking Chapter Select:
Complete the game once to unlock the Chapter Select option in the Main Menu.
Unlocking Fish Aquarium Screensavers:
Find the Meditation Shark Statues hidden around the game, sit on top of these Shark Statues and follow the on-screen bottom-left instructions to activate the Abzu screensaver (on PS4 press the Touch pad button to meditate or exit the screensaver). Once activated, you can press the left analogue stick on your controller to switch between the different fish in the “aquarium” section of the game you’re in, meaning you’ll find yourself following a majestic Manta Ray around until you see another fish in the reef. After you sit on/unlock these Meditation Statues, you can return to these non-gameplay screensaver sections from the game’s Main Menu to watch the fishes you’ve discovered.