Charles Ellicott Commentary Exodus 15:23

"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." — Exodus 15:23 (ASV)

The waters of Marah ... were bitter. The extreme bitterness of the springs at the southern extremity of the wilderness of Shur is witnessed to by all travellers (Burckhardt: Travels in Syria, p. 777; Robinson: Palestine, vol. i., p. 106; Wellsted, Arabia, vol. ii., p. 38, etc.). There are several such springs, that called Ain Howarah being the most copious, but scarcely so bitter as some others.

Therefore the name of it was called Marah. “Marah” means “bitterness” both in Hebrew and in Arabic. It appears to be a form of the root which we find also in mare and amarus.