I have a Remington 870 in synthetic stock. This was a very cheap shotgun -- about $289 I recollect. I bought it when I decided to give duck hunting a try because I was concerned that duck hunting would be hard on the shotgun and I didn't want to be freaking out about damaging a pretty wood stocked shotgun. I was concerned that I would splash water on the gun, drop the gun in the water by accident, trip and jam the barrel into the mud.

In point of fact, I have never dropped the gun in the water in 3 seasons of duck hunting in flooded timber and traipsing through knee deep water, sometimes frozen, sometimes through laid-over grassy stuff that likes to snag your feet, and up til now with no sling. I have never jammed the barrel in the mud. I have sometimes managed to get a couple of drops of water on the gun, but just a few drops. So the degree of risk of this is less than I had figured. I now have a shotgun sling (installed a stud on the synthetic stock and a stud on the magazine cap), so less chance of just dropping the shotgun while dancing through the marsh.

Of course, sometimes people may hunt ducks in a rain. I have never had this experience, but it is possible, maybe particularly a mist or drizzle or snow. Probably this is more of an issue for the metal parts than for the stock, however.

By the way, when I use the word "cheap" above I refer to the cost of the shotgun. It has been utterly reliable and serviceable in my use of it over three seasons. No signs of wear or rusting. I take very good care of it, even if it is inexpensive. I wipe it with an oily rag after every hunt. I used to clean the barrel after every hunt, both morning and evening, if I had shot. Now, however, I'm going to change my barrel cleaning regime and just clean it at the end of a hunting weekend. I think this is probably going to be just as effective at maintaining the health of the barrel as the more obsessive "after every hunt" regime.