Watering early in the day reduces evaporation loss and allows plant foliage to dry quickly. Wet foliage overnight may help trigger some diseases. Furrow irrigation, drip irrigation, or soaker hoses deliver water right at the soil surface and not on the leaves, so you can water most anytime. Avoid watering at mid-day, because evaporation losses are highest at this time. Use mulch to help retain moisture in the soil.
Mulches reduce the fluctuation of soil moisture and that helps the crop enormously.
But, remember, don't apply mulch until after the transplants have been going for 5 to 6 weeks. If mulch is applied too early, it will keep the soil cooler and plant growth will be slowed. However, when the soil is warm and plants have begun to grow, apply mulch and it will keep the soil cool during the summer and will also retain moisture. Too many fruits of tomato and bell pepper along with eggplant and watermelon grown in home gardens are lost each year to a condition called blossom-end rot. It is sad to see blossom rot erupt in a tomato row because you can't help the tomato once it's started.