If a backpack is what you desire, and something quite flexible in both wearing options as well as gear access, check out the Kata Sling series. I have one for when a backpack is the best option for whatever reason and it is quite ingenious:
Video bags camera bags Kata - Manufacturer of camera bags
Works as both a sling as well as backpack. Very easy to access gear while you are still wearing it, and comes with a nice raincover. It seems quite water resistant by itself anyway.