Here's the response I got from Gateway-
Hello and thank you for your interest in Gateway Feathers.
Wet feathers were a problem when guys shot off the shelf and used big
feathers.
Today's arrows are fletched with 4" parabolic shape or less and water is not
a factor. If your feathers get wet hunting simply shake the water out with
a sharp snap of the arrow before you shoot. Also, we have our waterproofing
which can only be applied after an arrow is fletched because otherwise the
feather will not stick to the shaft.
Sounds like this is less of an issue than I previously thought...