John Gill Commentary Numbers 33:8

"And they journeyed from before Hahiroth, and passed through the midst of the sea into the wilderness: and they went three days` journey in the wilderness of Etham, and encamped in Marah." — Numbers 33:8 (ASV)

And they departed from before Pihahiroth
Being forced by Pharaoh's army pressing upon them:

and passed through the midst of the sea ;
from shore to shore, as on dry land:

into the wilderness :
that part of it which lay on the other side, for still it was the wilderness of Etham they went into, as follows:

and went three days' journey in the wilderness of Etham, and pitched in
Marah ;
so called from the bitterness of the waters there, and which is computed to be forty miles from Pihahiroth.