Well, the called-out ones (Jew and gentile) are out of Egypt and going camping! So far, they only know that God is some nebulous entity with a strange mix of characteristics:
1. He keeps his promises to the patriarchs (the reason he gives for delivering them from Egypt)
2. He is interested in providing some experiential proof of his existence to Pharaoh, who says he’s never heard of this Hebrew god and couldn’t care less about him.
3. He wants to be known by his own people by experience–specifically, this experience. How often does he refer to himself with the epithet “the God who brought out of Egypt”?
4. He does some terrifying cataclysmic stuff–a lot of it just by enlisting the wind.
So, considering that, it’s weird that the first place they go after they sing the song of Moses (a song that pops up again Revelation 15), they were upset with Moses because they couldn’t drink the bitter water and they had nothing else to drink. One could ask, “What? After all that? Where’s their faith?” But I don’t really think they are deficient in faith at all. Maybe they just infer that a God who does these huge cosmic things shouldn’t be bothered with their comparatively insignificant problem of thirst. Can you imagine? “Excuse me, Mr. Sea-Splitting, Fiery-Hail-Sending King of the Universe, um… may I please have a glass of water?” Who would do that? Forget it! And from the people’s perspective, when they take that problem to Moses, Moses handles it with some help from God behind the scenes; they’re not chastised for taking it to Moses.
As the reading goes on, though, they do start to get in trouble–not for taking the constant food and water problems to Moses–but for not listening to Moses’ instructions when they do, which God takes as a personal offense. It’s as if some of them are saying, “Give me what I want from God, but don’t you dare overstep your bounds, Moses, and tell me what to do!” Poor Moses, though, much maligned, wasn’t overstepping his bounds. He was just trying to lead them into a daily relationship with God. He had some key information they needed, and they may have been interested to hear it, but they certainly didn’t want to have to do it.