Minecraft wiki entry for Things_not_to_do, associated text: In Survival mode, blocks take five times as long to break while the player is underwater. Therefore, there is a high risk of drowning while digging underwater, especially if the player is not close to the surface. Digging an air pocket under the ocean floor or placing magma blocks or soul sand to create a bubble column can produce a place where the player can slowly recover their air supply. It takes time to recover the breath meter so players should come prepared.