The Children of Israel reached Marah and complained about the bitter water. God shows Moses a tree to throw into the water, “and the waters were sweetened.” Immediately after this episode, the Torah says, “And He said, if you listen to the voice of Havayah your God and do what is right in His eyes and you will listen to His commandments and you will safeguard all of His laws, all the disease that I have put on Egypt, I will not put upon you, for I am Havayah, your healer.”
Note the connection between sweetening and healing. (This is the only appearance of the concept of ‘sweetness’ in the Torah and the main appearance of the concept ‘healing’ in the Torah). True healing sweetens bitter reality. The sweetening of the water is also called healing, as when Elisha the Prophet sweetened the bad water in Jericho, “Thus says God, I have healed these waters.” Elisha healed the bad water by throwing salt into it. The tree that Moses threw into the bitter water was also bitter, as in the words of our Sages that God heals the bitter with bitter. This is an important principle in healing: “Healing like with like”, as is especially predominant in certain methods of natural healing (homeopathy).
The commentators ask a question on our verse: If no disease will come upon us, what healing is involved?
Rebbe Simcha Bunim of Pashischa explained the verse in Isaiah, “And God smote Egypt, smiting and healing” as follows: The Zohar says that God smote Egypt, but healed Israel. In other words, all the plagues that smote Egypt were in and of themselves healing for Israel. They freed them of the husk of Egypt. This is good healing, but not perfect, for there is still the good side and the bad side – and each side is contracted within certain boundaries. But now, God says to us: “All the disease that I have put on Egypt, I will not put upon you" – I will not heal you by means of the disease that I will put upon Egypt. Instead, “I am Havayah, your healer”. I will heal you completely with expansive lovingkindness.
Initially, the focus is on separation of the good from the evil. The evil still exists and we must separate from it. But after the Splitting of the Sea and the Song at the Sea, we rise to a more lofty level of sweetening, “and the waters were sweetened.” The bitter reality is completely healed and sweetened and there is no need for any blow.
Complete sweetening is the Messianic destiny, as the Zohar says, “To transform darkness to light and bitter taste to sweetness”. As it says about the waters of the stream that will emanate from the Holy Temple, which will sweeten the bad and bitter waters, ““and the waters were healed.”
 Exodus 15:25.
 Exodus 15:26.
 Kings II: 2:21.
 Isaiah 19:22.
 Ezekiel 47:8.