You should be hydrating before, during and after exercise. As to how much depends on many factors. Below is a link to Coach Dean's article that will help give you better insight.
As for how soon to exercise after you eat is a personal preference. As for eating after a workout, if you ate that close to working out, you should be fine to skip your post-exercise meal, unless it was high intensity and then having a little carb and protein should suffice. www.sparkpeople.com/resource/fitness_artic