Then Moses ordered Israel to set out from the Red Sea, and they went into the wilderness of Shur. They went for three days in the wilderness and found no water. When they came to Marah, they could not drink the water of Marah because it was bitter. That is why it was called Marah. And the people complained against Moses, saying, "What shall we drink?" He 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.
There the Lord made for them a statute and an ordinance and there he put them to the test. He said, "If you will listen carefully to the voice of the Lord your God, and do what is right in his sight, and give heed to his commandments and keep all his statutes, I will not bring upon you any of the diseases that I brought upon the Egyptians; for I am the Lord who heals you."
Then they came to Elim, where there were twelve springs of water and seventy palm trees; and they camped there by the water.
The relationship between the Lord and his "chosen people" early in their Exodus was fraught with challenges and confusion. It took a while for the people to get used to their new spiritual Truth. By history, the Israelites had not been monotheistic; hence, Moses asked the burning bush just which god it represented. It was necessary to ensure that the people were aware of the Lord—his promise and his power.
In this passage, the Israelites go three days without water and then find the water at Marah to be undrinkably bitter. The Lord, through Moses, sweetens the water and promises the people a healthier, more bearable journey if they give him their complete faith. He then leads them where water, food, and shade were plentifully available.
I think our own spiritual commitments go through a similar phase. Our lives seem to be in upheaval as mortal challenges distract us from the relationship to the Divine that we have just awakened to. It's often the case that there comes a "miracle" of sorts—a sudden expression of the Divine active in our lives to encourage us to "stay the course."