Survival skills
A human being can survive an average of three to five days without the intake of water. The issues presented by the need for water dictate that unnecessary water loss by perspiration be avoided in survival situations. The need for water increases with exercise.
Infra-Worlders
The Infra-Worlders are a society of humans led by Captain that has lived for five decades in a gigantic submarine named the "Infra-World." These humans have been "evolved" so that their bodies can withstand the water pressure at 5,000 fathoms beneath the ocean's surface; as a result, they cannot live on the surface of the world.