2 Samuel 12:27 NKJV
 And Joab sent messengers to David, and said, "I have fought against Rabbah, and I have taken the city's water supply.
Joab was talking about war when he mentioned he had taken the city's water supply. This action would severely hinder the city's ability to defend itself, thus making it easier to defeat.
Spiritual parallel: In order for the enemy to destroy us spiritually, he must cut off our spiritual water supply. If he can hinder our access to the Word and the Holy Spirit, it will make it easier for us to be defeated. Therefore, we must carefully guard our water supply. We must be vigilant and respond to any attack on our interaction with the Word and the Spirit and fight back to protect our supply. In order for us to stay strong in battle and stay victorious, we must stay spiritually hydrated.