Had my old Safari Tuff out in some wicked down pours, and with the rain cap on everything was always dry. Great quiver, really outstanding product, just didn't fit my style of hunting.
When I moved to bow mounted quivers, I started keeping a vane'd arrow in the mix, but I also do the ziploc bag thing.
Someone, somewhere made a really cool waterproof fleece "bag" for lack of better terms that used a shock cord to hook it up to the bottom quiver mount and could cinch down to cover the feathers, but you could still pull them out with no problems.
Really slick setup, can't recall where I saw it on here, but would definitely like to see it again.