backback/sling pack

i have one of the original orvis sling packs and for as much as i have used it, it works well.  i believe i bought it at half price, possibly when orvis came out with the new and improved model.  what's nice about it is that i can slide it behind me and casting and playing fish is not encumbered by the pack.  also, i use it for spinning gear mainly.

I have three sling bags.  One was just retired, so actually two.  The retired one, I bought at Old Navy about 15 years ago on clearance.  It did it's duty for about 15 years of fishing.  It fit 3 of the 5 x 7 or whatever size that is boxes and a bunch of loose split shot and other containers.  Side compartments full of plastic grubs, ect.  I have a larger one that I use for salt water fishing, that carries two larger containers maybe 6 x 10.  I put my lures in one and terminal tackle in the other.  That one is a GAP bag, that I found at a thrift store...  The replacement bag for my Old Navy one, I found at Best Buy on clearance.  Pretty sure that I didn't pay more than $10 for any of them. 

My cheapos are all rather thick canvas type material, and have fished in torrential downpours and Summertime wades that have seen water.  I have never had any rusty hook issues with any of them (salt water gets a good wash after any fishing).  Even Fat Lenny never got wet when he used to tag along in my bag.  When fishing in a downpour, opening a 100% waterproof bag will let in just as much water as opening a non-resistant bag...  So, for waterproof, falling in or wading too deep would really be the only concern, IMO.  If it does get submerged while wading or fishing in a downpour, I make sure to take stuff out to let dry (most stuff is in containers inside of the bag).  Like someone mentioned above, slings are nice to wear on your back while fishing, then sling in front when needing to access.  You can tuck your rod in to free up your hands as well, or add some velcro straps, to strap to yourself (my wading jacket has this).  Those fly fishing chest packs are really nice looking and have contemplating buying one.  The ones that fold out to make a small shelf to free your hands up is a great idea.  I balance things on my sling bag to use as a temporary extra hand, but definitely not as nice as the chest packs.  I would imagine that fishing with a large one on your chest is annoying.  I know just leaving my sling bag on my front while fishing is.

simms
patagonia
sagbrush
orvis
seal-line
 
All have water proof (not water resistant) options... either you stuff gets wet or it doesn't, "waterproof" is the only way to ensure dry
and you pay to keep stuff dry.... price depending on closure system (zip vs roll top) and size
"When fishing in a downpour, opening a 100% waterproof bag will let in just as much water as opening a non-resistant bag"... yes.. but your stuff will already be wet from being in the rain with a non-water proof bag
 
You want to be able to close the back and squeeze it with no air coming out