Bitter things. No one likes bitter things. But bitter things are part of life. Life is not all honey and no bees. Roses do have thorns ... and all people face "bitter waters" from one degree or another.
The Marah story in Exodus 15 is especially significant. You see, in Exodus 14, God's people had just seen and personally experienced the greatest Old Testament miracle: the parting of the Red Sea and the drowning of Pharaoh and his armies. The people take the first 21 verses of chapter 15 to rejoice and dance and sing over the great power of God. Life is good, and God is awesome! Hallelujah!