Ghost fish are spectators in the game, accessible only by clicking the Spec button instead of the Play button. As a ghost fish, no non-ghosts can see nor interact with you. You cannot die from anything, whether oxygen, poison, temperature, pressure, or damage. You also cannot eat and have no experience, pressure, temperature, oxygen, or health bar.
You can live in the Deep, Shallows, and Arctic and can fly. You can go into the dirt and through islands an icecaps. By pressing and holding left click, you increase in movement speed.
You cannot choose a specific player to spectate. You have to try and follow them by moving fast.