Tomato plants let gardeners know when they are thirsty; the foliage begins to droop and wilt, particularly during the hottest part of the day. It is good practice to water in the morning, to provide the plants with the water needed to prevent them from becoming stressed (a condition that can lead to blossom end rot).
How often to water depends upon where the tomatoes are planted (traditional garden in the soil, raised bed, containers, or straw bales). Tomatoes in the ground have an ability to reach deep for water, lessening the frequency. Container and bale-grown tomatoes use the available water more quickly, and in very warm climates and/or dry spells, daily may be the rule. The type of soil is also a factor, as well as the draining ability. Heavy clay soil holds more water, and sandy soil retains less.
It is very important for the health of the tomato plants to water at the base of the plant rather than via overhead watering or use of a sprinkler. Wet tomato foliage acts as a magnet for various diseases that are spread by fungi, the most common of which are alternaria (early blight) and septoria. The fungal agents can splash up onto lower foliage from the ground, or blow in and adhere to the wet foliage from elsewhere.