Why did the Egyptians not send Yisrael to dig for water?
Oznayim la'Torah (citing Tevu'as Shor): This is a proof that the Shibud ceased already before Yisrael left Egypt. 1
And once the Makos began, Yisrael stopped working for the Egyptians.
What are the implications of "Vayachp'ru Kol Mitzrayim ? Mayim Lishtos"?
R. Bachye: On the one hand, it implies that they did not succeed in their quest for water and did not drink. On the other hand, one cannot go seven days without water, in which case it must mean that they drank ? They did not find water that was fit to drink, 1 but they did find salty water.
What is the definition of "ki Lo Yuchlu Lishtos mi'Meimei ha'Ye'or"
R. Bachye (citing R. Sa'adya Ga'on): "Meimei", as opposed to "Mei", 1 means sweet water that is fit to drink. 2
As in Bereishis 7:7 and Yeshayah 54:9. See R. Bachye and R. Chavel's footnotes.
R. Bachye: As in 23:28 and Bamidbar 20:8 and Divrei Haymim 2. 32:30.
