Hi, Tomatoes do best with about an inch of irrigation per week. Tap water is fine, but just be sure not to water the plant's foliage. It also helps a lot to keep your plants mulched for consistent soil moisture. This will help prevent tomato diseases from getting a foothold. The absolute best way to water tomatoes is with a drip irrigation system around the base of the plants, covered with mulch. To know when you have an inch of moisture, set a few coffee cans around the bed and measure the contents each time you water or when it rains.