Rosemary is considered a sacred/holy herb. It is used for protection, exorcism, banishment, blessing, consecrating, and a few other things I can't think of at the moment! Just google "Rosemary magick properties" and you should get a bunch of info.
If you seal it and refridgerate it while it sits for 3 days, you can drink it. It's best to use fresh rosemary in this case and bruise it (rub it between your fingers to produce the smell/oils).
An alternative would be to first heat the water to make an infusion with the rosemary, then refridgerate it when it cools without removing the rosemary for 3 days. This would be a more medicinal version. Rosemary is cleansing medicinally and magickally, so this would serve as a dual purpose.
You can use the 3 day period for water sitting out if you just plan to use it for blessing/consecrating/protection purposes (as a "holy water").
I personally prefer adding herbs to oil for annointing purposes. Fill a small glass container with a lid with the herbs you wish to use. Put it in a dark, cool area for 3 weeks. Then strain. I actually prefer not to strain because it looks pretty in the glass and I end up using it before it spoils. It all depends on what herbs/oils you're using and how well they keep.