that there is no crevice corrosion on the threads. If the nut was stainless and sitting wet with salt water then the eye may be junk. Considering the potential for major damage now is the time to replace it all.

A few years ago I had my chainplates tested. The guy said "see this" - I could barely see a thin line. He proceeded to bend 3/8 inch stainless with his bare hands. It was riddled with rust inside but you could barely see it on the surface.

Don't take chances. The risk is too high.