Why is it that drinking water hours before you masturbate increases the volume of semen?

Good question. I think it's because your body becomes more hydrated overall, so any glands responsible for secreting a watery substance (such as those in the male reproductive system) are able to work more efficiently and have less trouble getting water from the bloodstream. If you're dehydrated, on the other hand, your body will want to conserve water, so these glands will not have as much water available, and you'll ejaculate less.