Tomatoes need just the right amount of water when they are growing and it is extremely important to make sure that they are not over watered, which is indeed possible.
Usually you will want to use about Â½ inch of water for your tomato plants, as that will be enough to sustain them for the next few days. If you over water your tomato plants, you can very well kill them. This is why it is so necessary to be careful when you are watering them.
You should have your plants in moist soil, though not to the point where it is flooded in standing water.