To prevent rust, I suggest you remove the fore-end and give the metal parts of your gun a wipe over with either an oily cloth or a squirt of spray gun oil or WD40 as soon as you come off the marsh.
The only proviso is not to use WD40 on an old English gun, but it is OK on anything modern.
I also suggest you have a careful look at your gunslip, and make sure it is not the slightest bit damp inside.
Just to be sure, either put it in the airing cupboard or lay it over a radiator.
The sort of slip with a full-length zip is best in this respect, as it is easier to dry thoroughly.
A combination of salt, moisture and warmth (even the warmth in your car compared to that outside) is a deadly formula for encouraging rust.
In industry, a warm salt water spray is used to test rust resistance.