The rust concerns of zinc plated ammo are probably overstated. I have never had a problem with it even when I lived in humid South Carolina and Georgia. Here in Southern Nevada, it is a non-issue. Unless you are leaving your ammo in magazines that are constantly exposed to rain or humidity, it will likely never be a problem.
I've got 2 Silver Bear stories:
1. When I first got my CZ-82 as a carry gun about 5 years ago, there was no commercial JHP available for 9x18mm, save for Silver Bear (Hornady Crit Defense was about 8 months backordered). So I loaded up the silver bear in it. After 2 weeks of carrying it, I shucked all the rounds out of the mag to clean the lint out of it, and at that point I noticed it was white wherever my fingers had touched it and felt visibly different. I put some digital calipers on the case and it was now a few thousandths thicker (zinc oxide is less dense than zinc, so it occupies more space). I took the rounds out of the mag and set them aside, and switched back to brass-case FMJ. A week later I shot through the silver bear. Rounds I had never handled functioned fine, those rounds that had been loaded and carried around with me had a 20% FTE rate.
2. I took a carbine class in the summer where I was shooting silver bear because it was cheap, and I was hoping it would smell better than the cat-piss smell that Brown Bear makes (it doesnt, btw). When class was over I left the remaining partially loaded mags in my chest rig for a few months, and then the next time I went to the range I pulled them out and shot through them without looking at them. First round tore the case rim off and resulted in a stuck case. I manually removed the case, and sure as shit, it was all white. I unloaded my partial mags and looked at them: All white and crusty.
So in conclusion, water and humidity may do it, but I'm putting my bet on skin oils and salty acidic sweat being the bigger problem.
Most likely what you experienced was a result of the acidity of the water and duration of the exposure. The zinc works as a sacrificial coating to prevent the steel from rusting. Based on your experiences I am guessing you sweat a lot?
EDITED TO ADD:
Oh and the reason why Silver Bear and Brown Bear smell the same is because they are all made by the same company (Barnaul) and then marketed by DKG Trading and now Colt...http://www.dkgtrading.com/bear-co.htmlhttp://www.barnaul.co.nz/index.php?page=selection
The important factors that control the rate at zinc corrodes in atmospheric exposures are: http://www.keytometals.com/Article40.htm
The duration and frequency of moisture
The rate at which the surface dries
The extent of industrial pollution of the atmosphere.
Corrosion of Zinc in Water. The corrosion of zinc in water is largely controlled by the impurities present in the water. Naturally occurring waters are seldom pure. Even rainwater, which is distilled by nature, contains nitrogen, oxygen, CO2, and other gases, as well as entrained dust and smoke particles. Water that runs over the ground carries with it eroded soil, decaying vegetation, living microorganisms, dissolved salts, and colloidal and suspended matter. Water that seeps through soil contains dissolved CO2 and becomes acidic. Groundwater also contains salts of calcium, magnesium, iron, and manganese. Seawater contains many of these salts in addition to its high NaCl content.
Corrosion in dissolved salts, acids and bases.
Zinc is not used in contact with acid and strong alkaline solutions, because it corrodes rapidly in such media.
Very dilute concentrations of acids accelerate corrosion rates beyond the limits of usefulness. Alkaline solutions of moderate strength are much less corrosive than corresponding concentrations of acid, but are still corrosive enough to impair the usefulness of zinc
I wonder if you would have had a similar experience with lacquer or polymer coated ammo? Either way, all of these coatings are going to be better than just steel. I say use what you trust and what works for you. For CCW and serious defense purposes I use high quality brass cased HP/SP ammo because that is the best I can get.