Ex 15:22-25 (NIV) Then Moses led Israel from the Red Sea and they went into the Desert of Shur. For three days they traveled in the desert without finding water. 23 When they came to Marah, they could not drink its water because it was bitter. (That is why the place is called Marah.) 24 So the people grumbled against Moses, saying, "What are we to drink?"
25 Then Moses cried out to the Lord, and the Lord showed him a piece of wood. He threw it into the water, and the water became sweet.
NOTE: This is the second grumbling. The desert life is not going so good. Water is difficult to find. And water is an absolute necessity. One can only live a few days without water, and maybe less in the desert. The people have seen miraculous works of God in saving them, but again and again they complain to Moses (indirectly to God) about his provision. In essence the real argument is, "do you care about me? God, I know you are powerful, but do you care for me and my needs?" I believe this is always the struggle of faith when bad times come upon us. Some do struggle with God's power or existence (although their complaint against God suggests otherwise), but most people struggle with the idea of God, in his immensity, hearing and caring for one who is so insignificant in the grand scheme of things. In Hebrews 11:6, the writer makes the argument that real faith has two components, one, his existence, and two, that he is a rewarder of those who love him. In other words, he cares for and is working for the good of his children. Crises tend to bring the issue to the forefront. Real faith believes that God does care, and that he will provide -- he will bring good out of the situation. Although, God's definition of good and mine do not always coincide.