How quickly did they forget His deeds?
Radak: Three days after traveling from Yam Suf, they came to Marah, and they complained (Shemos 15:22-24). Had they remembered His deeds, they would have thought that He did not do these wonders and pass them across the sea to kill them via thirst 1 . They should have waited for what Hashem will do for them, like He did for them until now.
Malbim: Initially, Hashem gave to them manna and Slav. When they came to Refidim, He gave to them water and the Slav ceased. They thought that He cannot supply all three at once. This was their complaint at Kivros ha'Ta'avah - they did not believe in His ability.
Perhaps they did not forget, just they feared that just like most Jews died during Makas Choshech, now the majority will die! (PF)
What is the meaning of "Lo Chiku la'Atzaso"?
Rashi: They did not hope to him.
Radak: They did not wait for His counsel, and complained. His counsel was great and wondrous - He sweetened bitter water.
Malbim: His counsel (the reason He did not give to them meat - refer to 106:13:1:2) was "va'Y'ancha va'Yar'ivecha va'Ya'achilcha Es ha'Man" (Devarim 8:3) - in order to afflict them and know what is in their hearts to subdue their physicality and desire. They did not wait for this counsel.
