Why could the Israelites not drink the water at Marah?
The waters at Marah were bitter and therefore undrinkable, so the Israelites could not drink them. They had journeyed three days in the wilderness of Shur without finding water, and when they reached Marah, the water proved unfit for consumption.
Les explains that after Moses brought Israel from the Red Sea, where the Egyptians had drowned, they went three days into the wilderness of Shur and found no water. That many million people and all their livestock were in the hot Middle Eastern Sinai desert with no water. When they came to Marah, an oasis of some sort, the water was bitter and undrinkable. Les points out that this is a picture for new believers: shortly after their salvation experience, they will run into a bitter experience.
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.