It is interesting that in this Psalm it seems that the voice which is used to depict the speaker is that of God. More often than not it is the voice of the psalmist who is pleading to God for one reason or another. Here, instead, the author takes on the voice of God. This is true especially starting in vs.6 through the end of the psalm.
The reference to the waters of Meribah is a reference to Exodus 17 which describes the Israelites and their demand for water. The places Meribah and Massah mean quarrel and test respectively. This psalm reflects the quarreling and the testing that the people of Israel were responsible for in respect to God. It is a psalm of lament not of the people against God, like it usually is seen in the psalms, but rather a lament of God against the people, like is seen in the prophets and other places. I like the way which God trades places. It is absolutely justified in this situation.