cover stains

In addition to my Jon boat I also have a pontoon. The issue I'm having is that when it's stored outside with the cover on, there are black streaks of oxidization that drip down from the snaps that hold the cover on. I imagine it's a pretty common issue. Its a real pain to get this off of my boat. I don't think the snaps are stainless. Probably nickel coated brass. Will replacing these with stainless ones solve the issue? Are there any other things I can do to get this to stop? Thanks in advance!

Stainless snaps probably would be a good solution, as under rain conditions, it isn't going to oxidize like zinc or nickel does.