the people were thirsty, and God gave them to drink.
if Moses sinned, it is God's kindness that He did not cause all the people to suffer for it.
the same thing happens today -- i believe that there are true children of God within the Catholic church for example, though that church institutionally has false teaching in it. while Paul was in prison, he said that there were people preaching the gospel out of envy, but he rejoiced anyway that the truth was going out, regardless of the motive (Philippians 1:15).
so water is symbolic of the grace of God; as the Israelites drank from it even though Moses sinned in the way he struck the rock, people were refreshed by the gospel of Christ while Paul sat in jail, even though some men sinned in the way they preached it for rivalry's sake.
this idea reminds me too of how Balak hired Balaam to curse the Hebrews, but what transpired instead was that Balaam blessed them, because he could never curse what God had not cursed. Balak sinned, and Balaam should have blessed Israel from his own home instead of going to Balak and taking his pay, but the net result was still that Israel was blessed.