I have the Leatt, and carry quite a bit in there, probably too much.
I would say the main rear pouch is waterproof from a rain perspective, but if you fell over in a river, everything is going to get wet.
I does not double up as a kidney belt as the strap is actually quite thin.
It is very comfy to wear, but I am looking at some method of attaching it to my camelback as well to keep it "in place", because as I say, mine is little bit heavy and it tends to head downwards in rougher terrain, I only use mine on my WR though.