Then Moses made Israel set out from the Red Sea, and they went into the wilderness of Shur. They went three days in the wilderness and found no water.
22. So Moses brought. Moses now relates that, from the time, of their passage through the sea, they had been suffering for three days from the want of water, that the first they discovered was bitter, and that thence the name was given to the place. This was indeed no light temptation, to suffer thirst for three days in a dry land, and nowhere to meet with relief or remedy.
It should seem, it was with some difficulty that Moses prevailed with Israel to leave that triumphant shore on which they sang the foregoing song. They were so taken up with the sight, or with the song, or with the spoiling of the dead bodies, that they cared not to go forward, but Moses with much ado brought them from the Red Sea into a wilderness.
Commenting on Exodus 15:22-27
And when they came to Marah,.... A place in the wilderness, afterwards so called from the quality of the waters found here; wherefore this name is by anticipation: they could not drink of the waters of Marah, for they were bitter; and they must be very bitter for people in such circumstances, having been without water for three days, not to be able to drink...