Why did we pay for our water?
Rashi: We feared to draw water from the river due to the enemy, therefore we bought from them.
Ibn Ezra: Even water [and trees] that no one owns (they are Hefker), we bought them for a great price, because the enemy besieged the city.
Ri Kara: Normally one fills his flask from the spring and drinks, and cuts trees from the forest. Because the enemy "Tzadu [Tze'adeinu]" (4:18), we feared to go even "in our streets" (ibid). Therefore, we hired others to bring to us water and wood.
Palgei Mayim: Even when [other nations] were submissive under us, we drank even our water for money. We did not steal anything from them 1 ! Now our wood comes for a price. The goods that we did for them - we pay for the same wood that we carried on our necks when they chased us!
Why would it be theft to drink our own water for free? Perhaps 'our water' is from wells that Nochrim dug in Eretz Yisrael. (PF)