Sometimes symbols just slap us across the face with their meaning, and this is one of those times. (Ouch, water!) There's no two ways about it: water is a symbol of bonding in Stranger in a Strange Land.
At the beginning of the novel, Mike literally shares water with the people with whom he chooses to be "water brothers." Kind of like the sacrament of a church ceremony, the act brings together those who choose to partake in the event. In a way, it's fitting. We all need water to survive, and we're mostly composed of the stuff, right? So in sharing water, the water brothers are reminded of their commonality and grow closer.
Water is an even a bigger deal for Mike, though, because of how rare it is on Mars. When Jill gives him a bath at Ben's apartment (Chapter 8), he's overcome with joy. It's just a little reminder that the things we take for granted can hold great symbolic power for other people. One man's water is another man's… super important symbol?
And eventually, Mike realizes that water is just that—a symbol. Once he understands the symbolic nature of the water, it becomes less important to the ceremony of growing closer. It can be replaced with food, booze, and sex—especially sex, as Mike sees this as the ultimate bonding experience (check out our "Sex" section under Themes for more on that). As with all symbols, the importance is not in the thing itself but in what it represents. And, hey, if you could replace water with booze or sex, why wouldn't you?