Water-sharing is an idea from Robert A. Heinlein's Stranger in a Strange Land. It
is a Martian concept, equivalent to an oath of brotherhood. On Mars, water is scarce,
and (by inference from the text) water is hoarded. To offer someone water is an invitation to
take part in the water-sharing ritual and become water-brothers. Note that the Church of All Worlds
prefers waterkin for usage -- Heinlein's does not appear to have been meant to be sexist,
but rather was due to the weird nature of Martian gender. It was considered a growing-closer, and in the book
inspired a fierce loyalty in Michael Valentine Smith.
Water, elementally, is considered the element and perhaps the essence of life.
Sharing water translates to sharing life,
in the book with "all that Groks" -- divinity immanent in everyone. This obviously
ties in with the idea of "Thou art God/dess". Water cycles through our environment
and each one of us, and this ritual is a reminder of that which links life. According to both the book
and CAW, you aren't just linked to those you have shared water with, but those they too
have shared water with. In terms of a consciousness-sharing ritual, it can be compared with Wicca's Great Rite.
The form of the ritual in the book takes place between two individuals, we'll call them Foo and Bar.
Foo offers Bar water. Bar
can then accept. Foo drinks some of the water,
and then offers it to Bar with the words "Drink deep" and/or
"May you never thirst". Bar then drinks.
This practice is kept by the Church of All Worlds and also appears in various forms in the rites of
Reformed Druids of North America and Ar nDraiocht Fein. Careful observers might note a similarity
to the Christian Communion sacrament, particularly the form practiced by the Roman Catholic church.