It's really hard to say without looking at the plant. Let's try and help you out here:
Are the leaves of the tomato plant drooping before you water them?
If yes, is this during the hours of really bright sunlight? If you answered yes again, do your leaves get back to usual in the evening? If the leaves are getting back to their usual state in the evening, it's just a defense mechanism that the leaves are drooping. However, if your leaves are drooping and they stay that way, it's an indication that the plants need some water.
Also, most plants tend to protect their flowers and fruits, and so the leaves would be affected before the flower. Therefore, I can say with a high level of probability that this is not a watering issue.
Also, I am not sure how experienced a gardener you are. Do not take this to heart if I treat you as a newbie. But, the petals of a flower do drop off before the fruit can grow. If it's just the petals dropping off, don't worry.
Also, all the flowers on your plant should not be dropping at the same time. If all of them are falling off at the same time, it's definitely got something to do with an external phenomenon, watering could be one of them. A lack of nutrients in the soil usually results in a lower quantity of flowers and fruits or no flowers at all. It's unlikely that the flowers will grow and then fall off.
But during the flowering phase, it is recommended that you add phosphate and reduce the amount of nitrogen you feed your plants. Check out this article for more information on nutrition at different stages of tomato growth: http://www.yara.us/agriculture/crops/tomato/key-fa...