As usual, DFW is spot-on with his advice. When I was a tadpole, I was a pretty "serious" duck hunter...well, at least as serious as a 14 year old can be about stuff like that when he has no money
I have immersed shotshells many times and, of course, water gets in your duck boat...that 14ft. wood johnboat leaked enough that we often had to shoot a while, then bail, then shoot some more
...so spare ammo would often end up getting wet as well. Older, unwaxed paper shells were toast if they got wet. (Remember, we were not rich, and I would shoot damned near anything I could get) Waxed paper shells were much better, but would still swell if they got really wet and were not used for a couple of hours. Plastic was the shitz...it would shoot fine. (For the record, I have some brass case #00, and it has a paper seal and wad, so I think it would fare little better than waxed paper cased stuff)
"Wet" in this instance, however, is not the same as "wet" in something like NO! I think the best course of action would be to first seal the crimp and primer as noted by dfariswheel, then take a hard look at some sort of water-resistant packaging and next, a water-resistant pouch...or at least a pouch that will drain and dry fast. (Water resistance from the outside will translate into resistance to drainage if water does get in...ever torn one leg on a set of waders?) I think I would consider one of the vacum seal machines...like Seal-A-Meal, or similar...and package my shells, say 5 per pack so I would be able to keep as many as possible in a dry condition inside my pouch / storage.
The vacum seal gear is cheap enough that it might be worth a try at sealing some cheap ammo up and running an immersion test...it would certainly make for an interesting thread!