Since I'm a semi-professional photographer, this is something I've had to deal with for years. There are a good number of "waterproof" bags out there, but some are definitely better than others. Here is the one I am currently using https://umpqua.com/products/packs-and-bags/packs-bags/tongass-650-waterproof-waist-pack
Sage also makes a really nice one (get the large size) http://www.sageflyfish.com/fly-fishing-luggage/large-waist-pack/
The only issues I had with the Sage is the zipper. It is submersible, I've tested that on several occasions
![Wink ;) ;)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
But it is hard to close, and you will rip off the string zipper pull. I've had three of these bags, and it usually takes about a week. I just replaced them with a key ring, but it still can be hard to close. It is the last "pull" to get it over the gasket.
That being said, it is hard to beat a roll-top for being water tight, hence going to the Umpqua bag. One big thing to check out if you don't go with one of the two above, is to make sure the belt is fully attached to the bag and has straps on the sides to tighten it up. I had a Simms bag years ago that the belt basically just slid through the bag and it "rolled" on you all day. Both the bags i listed above will allow you to tighten the bag high on your waist and save your back. Carrying an SLR and maybe an extra lens, plus some fly boxes all day will kill your back if the bag doesn't fit tight.
They both also come with shoulder straps that allow you to take some of the weight off your back, especially when hiking and climbing mountains. Nothing like trying to climb and having 10lbs on your hips pulling you backwards. The strap allows you to put all the weight across your shoulder and chest.