"So Moses brought Israel from the Red Sea; then they went out into the Wilderness of Shur. And they went three days in the wilderness and found no water. Now when they came to Marah they could not drink the waters of Marah, for they were bitter. Therefore the name of it was called Marah. And the people complained against Moses, saying, 'What shall we drink?' So he cried out to the LORD, and the LORD showed him a tree. When he cast it into the waters, the waters were made sweet. There He made a statute and an ordinance for them, and there He tested them, and said, 'If you diligently heed the voice of the LORD your God and do what is right in His sight, give ear to His commandments and keep all His statutes, I will put none of the diseases on you which I have brought on the Egyptians. For I am the LORD who heals you" (Exo. 15:22-26).
Admittedly, it is difficult to feel much sympathy for the Israelite people when they complain. They had just been freed from generations of slavery by the all-powerful, true and living God, but they murmur here again a second time (cf. 14:11). Did they need water? Absolutely! But where is the respect and trust for God and His servant Moses? What happened to their song of praise? How could a people witness so many miracles and God's deliverance through the Red Sea on dry ground and sincerely worry about life's necessities--as if the LORD would not provide? To inquire about water is one thing, but to complain is another matter entirely. Regardless, God is patient with them and provides a solution. Although their faith is weak, at times we behave similarly. May disciples of Christ never forget that God has promised life's necessities to those who seek first the kingdom of God and His righteousness (cf. Matt. 6:33)! May Romans 8:32 give us comfort and strength in difficult times - "He who did not spare His own Son, but delivered Him up for us all, how shall He not with Him also freely give us all things." God does not forget His children!
The text mentions that God there made "an ordinance for them and there He tested them" (15:25). The next verse explains the divine regulation that was given. It can be summarized as follows: "Be diligent to listen to and obey Me (i.e., God) and in so doing I will bless you with good health." They would not suffer "the boils of Egypt, with tumors, with the scab, and with the itch" (Deut. 28:27). Ultimately, God has always blessed those who obey Him. Such is still true today (cf. Rom. 8:28). Books have been written about the health benefits to following God's word. But the blessings in obedience to God go much deeper than the physical. Would the Israelites be attentive to God's word and obey Him? Their history as told in the Old Testament answers that question with a resounding "no", unfortunately.
"Then they came to Elim, where there were twelve wells of water and seventy palm trees; so they camped there by the waters" (Exo. 15:27). Here the LORD provides for their needs again.