A coating of beeswax is the only thing I know that will work...that's natural and non-toxic. Of course, you can come up with all sorts of beeswax, pine pitch, and tallow recipes that can be mixed up and applied to the rawhide.
I've tried making a quiver from rawhide and the thing was kinda noisy. The hide crinkled loudly and also amplified the rattling sound of the arrows within. I think you would need some pretty thick hide to keep the noise down.
Water doesn't affect rawhide too badly as long as it's not submerged. It should be fine in the rain for about 15-30 minutes, without waterproofing.