Airswimming is a glitch in Sekiro: Shadows Die Twice which lets the player underwater swim outside of its intended water volume/plain, causing the player to underwater swim indefinitely and freely in and out of bounds. The indefinite swimming activates if the player dives underwater and enters an area outside of intended boundaries of the game. The intended behavior to negate an underwater swimming state is the game expecting the player simply returning back to surface which is limited by walls and boundaries but since the player enters an out of bounds state, this swimming state is never switched off. The swimming state only resets if the player dives out an existing water plain again, voids out by touching a killbox or if a cutscene is triggered overwriting this state and returning the player back to solid ground.
Airswimming is useful for multiple different reasons and can be done in multiple area's where a player is able to swim underwater outside and out of the intended boundaries of the game.
Airswim - Sunken Valley/Guardian Ape to Senpou Temple