The letter W stands for Water. Water is essential for human survival. Humans can usually only survive up to 3 days without water, although some have lived up to eight days without it. The human body is 65% water. It is recommended to drink at least eight cups of water per day.

Sr E G White says in The Ministry of Healing:
In health and in sickness, pure water is one of Heaven’s choicest blessings. Its proper use promotes health. It is the beverage which God provided to quench the thirst of animals and man. Drank freely, it helps to supply the necessities of the system, and assists nature to resist disease.

 She has also written:
Thousands have died for want of pure water and pure air, who might have lived.... These blessings they need in order to become well. If they would become enlightened, and let medicine alone, and accustom themselves to outdoor exercise, and to air in their houses, summer and winter, and use soft water for drinking and bathing purposes, they would be comparatively well and happy instead of dragging out a miserable existence.

Water is essential to human life and is linked to the story of the Woman at the Well. There is a children's song that goes:
A woman came to fill her water pot,
down at the bottom of the well.
But Jesus gave her water which was not,
down at the bottom of the well!
She went away,
Happy and and Gay,
back to her home,
never to roam again for 
Jesus gave her water which was not,
down at the bottom of the well.

The story tells how the woman came for water at the well. Jesus was there and they started talking. At first the woman was skeptical, but Jesus convinced her she was right. Once she realized who Jesus was, she went away very excited and told all her friends. 

In the same way we are to partake of earthly and heavenly water.