Why was the water bitter at Marah?

Answer 1

This was a stop for the now freed Israelites who had just witnessed the parting of the Red Sea, their passage safely through it, and the destruction of Pharaoh's army and chariots. There are bitter or unfit to drink watering holes around the world (generally because the water flows over or through alkaline materials, which may answer the question to the extent you care right there) and The Bible tells us the name Marah means bitter so it was a known place as the quality of the water was too. See this verse:

Exodus 15:22-23

New International Version (NIV)

The Waters of Marah and Elim

22 Then Moses led Israel from the Red Sea and they went into the Desert of Shur. For three days they traveled in the desert without finding water. 23 When they came to Marah, they could not drink its water because it was bitter. (That is why the place is called Marah.[a])

Footnotes:

1. Exodus 15:23 Marahmeans bitter.

God does not act in vain so His bringing the Israelites to this spot served His purpose. Soon the peoples began to murmur against Moses and God. So God once again displays His awesome power and protection for His children by making the water drinkable. Soon thereafter, the people would continue on complaining about no food, so God would send manna from heaven to teach them about the Sabbath rest captivity caused them to forget. Yet, from their freedom onwards, the people continued to complain and do things their ways - the way of man. See related link below: