Ocean or Big Oceans are Zones where mostly the entire surface consist of seawater, Water blocks.
The Ocean Zone contains many naturally occurring (spawning) animals, such as:
-- Orca Whales
-- Sea Turtles
-- Clown Fish
In designer mode a lot more animals can be spawned using the Spawner Blocks, like:
In Designer Mode the sea level can be adjusted up and down , and those simulate global warming or change of the tidal current. See the ocean level change here
The Ocean surface level is at default set to be 127 blocks or 63,5 meters.
In Survival, Big World and Designer Mode (when gravity and fluids is switched on), the oceans will spread and flood and empty space below the water level, as well as transform and erode the landscape
Water from the ocean can change from being replicating and saltwater to being freshwater and not replicating (not flooding), for more details see Water.
Initially, the height of the world used to be only 256 blocks, so on old maps water level used to be 70 blocks.
After the update increasing the world height to 384 blocks, more blocks were added below the bottom of the world so the water level increased to 127.