This seemeth to refer to the state of the Jews in Babylon, where it is probable their adversaries made them buy both water and wood, which in the land of Canaan they had plentifully, and without any further charge to them than fetching the one, and cutting down and bringing home the other.
Related Commentaries of Lamentations 5:4
Lamentations 5:4
4 We have drunken our water for money; our wood is sold unto us.