I just went hunting yesterday in the Palmico sound in the NC outer banks for divers. Coming home today not even 24 hours after my hunt I opened my gun case to clean my 870 that I took with me. To my surprise it has already shown major signs of rusting all along the barrel. Some back ground info on the gun. It's a wooden 870 magnum thats about 2-3 years old now. I have never had issues with this at all and this is totally new to me. I'm pretty sure the steel on the barrel isn't blued as it is not smooth like blued steel. Anyways after the hunt we just packed up and left and decided we would clean our guns the next day (today) we didn't wipe them off or anything. I didn't think anything of it as the most my gun was exposed to the water was on our boat trip to and from the blind. In addition to this I now am unable to unscrew the castle nut (I think this is what its called its the piece you unscrew to be able to remove the barrel and the slide when you disassemble it.) and this is usually a piece that comes off very easily. So now I'm in a position where I cannot even fully disassemble my gun. I researched this a little and its a common issue apparently so I put some wd40 on it to soak in over night and see if this will help ease my problem. I was wondering what you all would recommend as far as rust removal and this other castle nut issue. I used solvent to remove most of the rust but there is still some really ingrained in the metal. I also found it quite odd that my gun would react to the brackish water that fast after being exposed. Is this normal? I also gave my 870 a thin coat of rem oil using the rem oil wipes in the meantime till I can figure out what to do. Also for future reference what can I do to prevent this from happening again next time I hunt in water that may have some salinity to it. Thanks for any answers or help.

24 hours is a long time to have salt spray on a blued gun. nothing out of the ordinary here. After you get it cleaned up, keep it wiped down with good oil. Do NOT use Wd40 in the action. If you already have, get some spray degreaser and get it out of there. WD40, time and cold will eventually gum up your gun so bad it wont work.
The way to prevent this from happening again is to wipe the exterior down prior to the trip and then after.

I have hunted ducks in brackish water for 40 years.

870s with the rough finish rust easily like that.  But it's just surface rust and won't hurt anything.  Just rub it off with an oil rag.

The caps used to have a slot in the top so you could use a flat bladed implement to unscrew a stubborn one.  But for the new cap, I would wrap a cloth around the cap and try to take it off with channel locks.

Motor oil.  Not fish oil.

Use Kroil to penetrate the rusted cap