Since water is scarce on Mars, it holds a very religous significance. When a Martian shares a drink of water with another Martian, they are forever water brothers, and, between them, what is yours, is mine hold very true.
Water-sharing is wholly Martian, and the concept is almost unexplainable in English. Water-sharing has the religous signicance of baptisim, and once you've shared water, you have agreed to trust the other person with everything, and that you will always be trustworthy to your new water-brother.