If you develop a simple theory about how much water people drink in the summer relative to how much they drink in the winter, will your theory be correct for all people? If not, is your theory worthless in describing human behavior?

No, my theory will not be correct for all people because not everyone is the same and everyone drinks different amounts of water. My theory, however, is not worthless in describing human behavior because it does describe the general human behavior, but it does not necessarily apply to every single individual.