Even the best quality 316 stainless will discolour eventualy in saltwater. It takes work to look after metal deck fittings. On very long crossings, 4/5000 miles, we used to use a polymer based coating to protect the stainless against salts sitting in even the tiniest of crevices in the surface of all metals.
Here is a quick guide from Grundfos pumps that confirms this. http://standards.nsf.org/apps/group_...Resistance.pdf