Why couldn't Israel drink the water at Marah?
Israel could not drink the water at Marah because the waters there were bitter, rendering them undrinkable. After three days in the wilderness of Shur without water, they arrived at Marah, an oasis of some sort, but its water was not fit for consumption.
Les explains that after Moses brought Israel from the Red Sea, they went three days into the wilderness of Shur and found no water. When they came to Marah, an oasis of some sort, they could not drink the water because it was bitter, meaning undrinkable. Les points out that in just a little while after their salvation experience, they were going to run into a bitter experience, illustrating that new believers often encounter bitter experiences soon after salvation.
And when they came to Marah, they could not drink of the waters of Marah, for they were bitter: therefore the name of it was called Marah.