Sforno on Exodus 15:26/ספורנו על שמות ט״ו:כ״ו

1אם שמוע תשמע לקול ה' אלוקיך, to accept the statutes which would follow as a way of life, not merely as emergency regulation in the desert, and if from now on they would הישר בעיניו תעשה והאזנת למצותיו, וגו, carefully observe G’d’s commandments endeavouring to live according to their understanding of what is pleasing to Him, then they would be permanently spared any of the afflictions that G’d had brought upon the Egyptians. The implication was that betraying G’d’s trust would have painful consequences. The entire experience is a reminder of the manner in which the Rabbis relate to a potential convert to Judaism. One does not present Judaism in rosy attractive colours so as to encourage him to convert, but on the contrary, one apprises him first and foremost of the עול המצות, “the yoke of the commandments,” telling him of all the things he would have to forego by joining the Jewish people, i.e. the Jewish religion. Moreover, one advises him that whereas up until now he was able to eat certain foods, such as chelev, not only would he no longer be able to eat these foods, but violation of the commandment would carry the penalty of karet, loss of entitlement to his afterlife, etc. Only after he has duly absorbed this information does one proceed with teaching him the intricacies of Judaism. What happened at Marah was preparatory to accepting Judaism at Mount Sinai. If the people had not gone through this process it is doubtful that they would have said נעשה ונשמע, “we will do, now let us learn G’d’s Torah,” when the time came for this a few weeks later.

2כי אני ה' רופאך, the point of all this is that once you have accepted the statutes and then fail to honour your commitment you will be subject to punishment by G’d. These punishments will be in the nature of being self-inflicted as all of G’d’s commandments are designed to make your life on earth and beyond a healthy life. Failure to honour your commitment will expose you to all the negative influences which abound on earth. I have shown you that I can cure harmful waters, waters which humans cannot drink. This is only a sample of what My commandments and observing them can do for you. We, today, do not know what kind of wood or tree G’d told Moses to throw into these waters. The Torah deliberately did not reveal this information. Even if all the trees in the world would be the kind that G’d told Moses to throw into the water, and even if we would throw all this wood into only a minute quantity of water the result would be nil. On the other hand, if, by the grace of G’d, one throws even a minute quantity of such wood into even an entire ocean, the waters of the ocean would suddenly become drinking water. [my addition. Ed.] Leviticus 20,26 states clearly that G’d separates the Jewish people from the surrounding nations. Failure to honour commitments results in “disease,” i.e. the defilement of one’s soul. As a result, such a person deserves his punishment.