Although I have not tested it by dunking it in the water, you can zip it shut and squeeze it as hard as you want and air will not come out. So I believe it to be airtight, thus waterproof. I have only heard good things about this company and would highly recommend their products.
I have a friend who guides 5 months in the Bahamas every year and uses a Sagebrush fanny pack. I think he told me that he’s actually snorkeled with it and it didn’t fail.