Subnautica features four different Game Modes. The player can select a Game Mode when starting a new save file; a save file's Game Mode cannot be changed after creation. Each Game Mode imposes different restrictions (or lack thereof) on gameplay.
To stay alive, the player must manage their health, oxygen, food, and water levels. If the player runs out of health or oxygen, they die and lose all of the items in their inventory. If the player runs out of either food or water, they will begin to lose health until they die. The player can return to where they died to attempt to reclaim their lost items. The player can "secure" their inventory by entering Lifepod 5, a Seabase, or a Cyclops; if the player dies with a secured inventory, they will keep all the items from the last time their inventory was secured and will respawn in the last place they secured their inventory.
Similar to Survival Mode, but the player does not need to maintain their food or water levels.
Similar to Survival Mode, but restricts the player to a single life only; if they die, the game ends, forcing them to completely start over from the beginning. The PDA will tell the player when they have low oxygen, despite the description saying otherwise. This is most likely a bug.
This is a sandbox mode for players who want to freely explore and build. The player cannot die and does not have to worry about health, oxygen, hunger or thirst. Materials and Blueprints are not required for crafting, and vehicles and Seabases do not require Energy to operate and cannot be damaged. The player can make story progress in this mode, despite the description saying otherwise.