What is "Motza ha'Mayim"?
Rashi: It is the source from which it flows.
Why did he say to put salt in the flask?
Rashi (from Tanchuma Beshalach 24): This was a double miracle. Salt normally makes water bad (not proper to drink). Radak - the same applies to "va'Yorehu Hashem Etz va'Yashlech El ha'Mayim" (Shemos 15:25). It was bitter wood; it was a double miracle.
Malbim #1: The salt was the reason why the water was bad. It was transformed to be the cure, just like "va'Yorehu Hashem Etz va'Yashlech El ha'Mayim" (Shemos 15:25); Hardufni (bitter wood) sweetened the water (Mechilta Beshalach, va'Yasa 1). Yeshayah smeared a fig cake on a boil, and it cured (Yeshayah 38:21). This was opposite nature. Natural salt makes water bitter; miraculous salt sweetens it.
Malbim #2: The water hints to learning foreign matters with heresy and foreign values - 'lest you be exiled to a place of bad water, and the Talmidim will drink and die' (Avos 1:11), i.e. death of the soul. Through this came the malady of the water and physical death, Midah k'Neged Midah. Elisha cast salt, which hints to an enduring intellectual matter - learning Torah, about which there is Bris Melach. He cast it to the source of these teachings, so their source would be in Kedushah of Torah and Emunah, and the water was cured.
How will this stop the deaths?
Radak: The deaths were due to the water.
