After three exhausting days wandering in the desert without water, the Israelites finally arrive at a water source, only to discover to their shock that it is unfit to drink [ביאור שטיינזלץ]. Overcome with sadness and worry, they turn to Moses. This event marks one of the ten times the ancestors tested God in the wilderness [תורה תמימה].
The nature of their reaction is highly specific. Rather than a fierce public argument or an aggressive demand, their complaint takes the form of a quiet, internal murmuring born out of sadness [רש״י, מזרחי, הכתב והקבלה, ביאור יש״ר]. The concept behind their grievance is tied to the idea of seeking shelter. In their deep distress, they looked for a protective shield, unloading the heavy burden of their sadness by casting the blame onto Moses [רש״ר הירש].
The commentators offer different perspectives on the severity of the people's actions. On one hand, they face criticism. Since the Israelites knew Moses acted entirely by God's command, a complaint against him was effectively a complaint against God Himself. They should have trusted the God who had already performed miracles for them and simply prayed to Him for water. Their choice to murmur instead reveals that their faith was not yet entirely whole [מלבי״ם, ביאור יש״ר].
On the other hand, some perspectives defend the Israelites. Their question about what they would drink was not an act of defiance, but simply a natural expression of a basic need. They did not aggressively demand water; they merely pointed out the grim reality of the bitter water before them. Because of this, God was not angry at the core request, but only at the complaining manner in which it was delivered. Consequently, this incident is not counted among the severe sins of the desert generation [אור החיים, ברכת אשר בשם אברבנאל]. Still, asking what they would drink carried an underlying accusation against Moses for leading them to that specific place [קאסוטו].
Interestingly, at that exact moment, the people were not yet suffering from actual thirst. Their reaction was driven by anxiety about the near future. They worried about what would happen when the water supplies in their own vessels finally ran out and they faced true thirst. Because the crisis was still impending rather than immediate, the complaint did not come from the entire nation at once, but only from a portion of the people [העמק דבר, ביאור יש״ר].