Marah in Smiths Bible Dictionary

(bitterness), a place which lay in the wilderness of Shur or Etham, three days journey distant, Ex 15:23; Nu 33:8 from the place at which the Israelites crossed the Red Sea, and where was a spring of bitter water, sweetened subsequently by the casting in of a tree which "the Lord showed" to Moses. Howarah, distant 16 1/2 hours (47 miles) from Ayoun Mousa, the Israelites' first encampment, has been by many identified with it, apparently because it is the bitterest water in the neighborhood.