The Air Stations are temporary landmarks that the player can only access on their first trip into the Zahhab Region's Depths area (the Twin Crevasses) in Endless Ocean: Blue World. They have no physical appearance, but the player can see where they are by locating the glimmering swirls hovering in the water above them (the swirls that indicate a quest item or location).
The purpose of these stations (supposedly large tanks filled with a usable, breathable air supply that have been submerged into the Depths prior to the player's trip there) is to allow the player to refill their rapidly depleting air supply during their first expedition to the deepest parts of the ocean, as the high-pressure environment is new to the player character and their body requires more oxygen at depths such as these. When the player selects an air station to interact with, their air supply will be refilled to its full capacity, and they can continue exploring.
Though these stations are necessary the first time the player goes to the Zahhab Region Depths, after this they obtain a special mixture of air (called “Tri-Mix” in the player’s ‘key items’ tab of their inventory) that allows them to breathe normally at such depths. However, because the player is still under a lot of water pressure, they will still use the maximum amount of air they can use. As a way to remedy this, the player can buy regulators and larger air tanks from Nancy in order to extend their breathing time.