The Torah reading for the seventh day of Pesach describes the miraculous splitting of the sea and the song that Klal Yisrael sang afterword. One would imagine that the reading would end with the words of Miriam exhorting the women to praise and sing to Hashem (Shemos 15:21). And yet we read five more pesukim which depict how after kriyas yam suf, the Jewish people traveled for three days in the desert without water. Finally, they come to Marah, but they are not able to drink the bitter waters there; so they complain, and Hashem shows Moshe how to sweeten the water. Why do we end with this seemingly extraneous passage? It does not seem to be a fitting conclusion to kriyas yam suf and the shiras hayam.
What is even more intriguing is how just three days after witnessing the great miracle of kriyas yam suf, the Jewish people complained about water. Chazal comment that even a maidservant saw divine revelation at the sea that prophets were not able to behold (Rashi, Shemos 15:2). The Ramban (Vayeira 18:1) explains that they merited such a vision because "they had faith in Hashem and Moshe, His servant" (Shemos 14:31) and they jumped into the sea. How did Klal Yisrael lose their faith so quickly that they complained about the lack of drinkable water?
The Ramban (Ha'emunah V'Habitachon, 19) writes that when a person experiences a spiritual awakening, he should concretize his feelings by doing something substantive, to translate his emotion into action. He suggests that this is the deeper meaning of the posuk, "If you arouse or awaken the love until you desire it - ad shetech'patz" (Shir Hashirim 2:7). Shlomo HaMelech is advising Klal Yisrael that when you have an intense feeling of ahavas Hashem, you should turn that emotion into something substantive, like an object (a chefetz); crystalize your feelings and transform them into meaningful action. In that way, your emotion will not be just a fleeting passion, rather it will leave a lasting and permanent impression.
Since Klal Yisrael never concretized their emotional experience at the yam suf into something substantive, their spiritual high evaporated in just three days of wandering in the desert. Hashem responded by giving them mitzvos - "There he established for [the nation] a law and a rule - chok u'mishpat" (Shemos 15:25). Rashi explains that chok refers to the mitzvos of Shabbos and para aduma. Why was Shabbos the first mitzvah that Klal Yisrael received in Marah?
The mitzvah of Shabbos is designed to strengthen our emunah in Hashem (see introduction to Mishna Berura). One who observes Shabbos testifies to the fact that Hashem created the world (Yisro 20:11) and that He took Klal Yisrael out of Mitzrayim (V'eschanan 5:15). This is also the purpose of the mitzvos associated with the yom tov of Pesach in general and the seder night in particular - to reinforce the idea of hashgachas Hashem, that Hashem is constantly orchestrating the events of history and of our personal lives (see Ramban, end of Parshas Bo).
Klal Yisrael witnessed the ten plagues in Mitzrayim. They saw Hashem's omnipotence, His total control over Pharoh and his people. This heightened sense of emunah in Hashem reached its pinnacle at the yam suf when they saw the precise divine retribution, the midah k'neged midah, that Hashem meted out to the Egyptians for their persecution of Klal Yisrael (see Netziv, Shemos 14:31). And yet, because these feelings of emunah were not grounded in mitzvos, in substantive actions, they were not lasting. That is why Hashem gave them the mitzvah of Shabbos which reinforces the concept of emunah, so that by observing Shabbos they would be able to crystalize their feelings of faith into something real and give them a sense of permanence.
Thus, the story of Marah is actually a fitting conclusion to kriyas yam suf because it was at Marah that Klal Yisrael received the mitzvah of Shabbos, the necessary chefetz, the vessel, through which they would be able to concretize the emunah they felt at the yam suf and transform that emotion into an enduring legacy.
It has been a little over a year since our emunah has been tested and hopefully strengthened by the COVID pandemic. Chazal say that thunder was created to straighten the crookedness of the heart (Brachos 59a). Hearing a sudden clap of thunder shocks a person and it reminds him of Hashem's omnipotence. The same is true with a global pandemic. But the story of Marah teaches us that the way to ensure that our heightened sense of emunah continues to inform and guide our lives is by concretizing it with a renewed commitment to Torah study and mitzvah observance. When we turn our spiritual emotion into something substantive, we infuse it with a sense of permanence, and we elevate ourselves in the process.