In some movies, tv series, games, etc. you can find underwater stations.
The on in the picture is from the series LOST.
These kind of stations usually have a "pool" like area, meant to enter the station via some kind of u-boat.
Now all of us know that the force of water pressure can be quite deadly. Once you dive down to a certain depth, the water will quite literally crush your body.
Now imagine you were swimming in a pool like this, within a station on the bottom of the ocean. What would happen? Would you die, if you fell into that pool? Could you swim safely on the surface but should just avoid going for a deeper dive in this "pool"?