Molasses does amazing things for plants, from being a natural insecticide, to providing complex sugars for microorganisms in your soil, to adding potassium and other nutrients. Potassium happens to be very beneficial to cannabis plants in particular, as cannabis requires a plethora of potassium during the important flowering stage.
Unsulphured is the most important requirement for your molasses. Personally, I prefer the Wholesome Sweeteners Unsulphured Organic brand (get that dankness at whole foods stores or buy now at Amazon here) for a couple of reasons. One, I don't want to have anything inorganic in my soil, the organic, non-gmo, and fair trade are not included with all molasses. All molasses are not created equal. Second, I want the highest quality possible, with the highest density of nutrients, for the most benefit. The cheap molasses (stay away from the grandma brand - no offense grandma) generally has less nutrients, and therefore possibly less impact.
How much molasses should you add per gallon of water?
1 tablespoon per gallon is the recommended dosage. You can go slightly heavier perhaps, but I would be cautious, 1/2 tablespoon or less and you likely won't see much benefit. Warning: be prepared to clean up some muddy looking water (the molasses turns all of your water darker)