Jamieson, Fausset and Brown on Exodus 15:23

23. when they came to Marah, they could not drink of the waters--Following the general route of all travellers southward, between the sea and the tableland of the Tih ("valley of wandering"), Marah is almost universally believed to be what is now called Howarah, in Wady Amarah, about thirty miles from the place where the Israelites landed on the eastern shore of the Red Sea--a distance quite sufficient for their march of three days. There is no other perennial spring in the intermediate space. The water still retains its ancient character, and has a bad name among the Arabs, who seldom allow their camels to partake of it.