According to Exodus 15, the Israelites found bitter water at Marah.
After crossing the sea, the Israelites traveled through the wilderness for three days without finding water. At Marah they finally discovered a source, but they could not drink it because it was bitter. Moses cried out to God, and the narrative says that a piece of wood was thrown into the water, making it drinkable.
The name Marah is related to bitterness, which explains the place-name in the story. Marah is not the same location as Elim, the next stopping place, where Exodus says there were twelve springs of water and seventy palm trees. It is also distinct from Rephidim, where the Israelites later faced a shortage of water and encountered Amalek.