It's only the $2 reservation fee and your first night rate that isn't refundable. So just make all of your site preferences on the water. If they don't have one, I would assume you could convince them to refund everything since they can't provide you what you want. Or wait another 11 days and call them and make your reservation and be assured you get what you want.
However, I don't think you have much to worry about. While it's a great campground, three years ago I made waterfront reservations in June or July for the first week in August and got the waterfront site we wanted.
Both Brimley and Aune Osborne are great campgrounds.
