When a gun becomes submerged in a flood there are several things to consider. How valuable or rare is the gun? How contaminated was the water? Is the gun still under water? If it’s still under, it is likely better than pulling it out and letting oxygen get to it, if it is going to sit for a few days. It is very important that guns don’t get taken from a flood and left out to ‘air dry’ as all the internal springs and parts will be ruined.
The best thing to do is pull it out, dry it off and rinse any dirt and other debris off it, then immediately submerge it into some type of oil. Diesel is a cheap and plentiful oil that will work for a while. As soon as possible the gun should be disassembled, and each part cleaned and inspected. Wood and plastic will be considered later. If the gun has a significant collector value re-bluing it could lower the collectability and value. If possible, high value guns should be restored by an appropriate gunsmith specializing in restoration work. Depending on the amount of damage to the finish a choice needs to be made on whether to refinish or attempt to preserve what finish is there. That will depend on each gun and its value and condition. Many gun finishes are some type of ‘rust’ that is controlled and then blocked from advancing by layers of oil or wax that block any more oxygen getting to the metal. Refinishing requires the removal of the old oxide layer to fresh metal and then a new oxidation process to be done. Typically, it takes several times through the oxidation process to get the finish that is desired. Once the finish is dark enough oil is put on to stop further oxidation and preserve the finish.
Plastic parts can likely be cleaned with a mild detergent and soft brush. Solvent should not be used on plastics.
Wood furniture may end up growing mold or mildew after prolonged submersion. A solution of mild Clorox will kill most molds but will probably remove the finish of the wood. A reapplication of either stain, wax, oil or the finish of your choice can be done after rinsing and allowing the wood to dry.