Why did Moshe need to force Yisrael (Va'yasa Moshe) to travel?
Rashi: Because the booty of the sea exceeded that of Egypt, as the Pasuk hints in Shir ha'Shirim (1:11). (Everyone left Egypt with 90 laden donkeys. How could they take more than this from what was on the horses? This is fine according to the opinion that there were 1500 Egyptians for each Yisrael (refer to 14:7:2:3*). Or, the booty of the sea (silver, gold and gems) was worth more than that of Egypt, which was mostly clothing.
Why did they complain about not having water only after three days?
Da'as Zekenim #1: They took water from Mitzrayim in Kelim, and after three days it ran out. They should have consulted [Moshe] before complaining.
Da'as Zekenim #2, Hadar Zekenim, Rosh: 'Water' refers to Torah. After three days without Torah, they rebelled. 1
Da'as Zekenim, Hadar Zekenim, Rosh: This is why Chachamim enacted to read the Torah on Monday Thursday and Shabbos, lest there be three days without Torah.