I have stainless steel handrails that are rusing below the water line. What could be the possible causes of this type of corrosion or staining.

Welcome to the forum Stainless steel is not rust PROOF. It just holds up better against it than regular iron or steel. Depending on the quality, stainless can exhibit signs of rust pretty quickly when constantly exposed to moisture.

If the corrosion seems excessive for the time spent in the water, there are several possibilities:

1) A trichlor tab floater is being, or has been used, and the floater has been too near the rail.
2) Water chemistry is way off. Especially low pH and/or high FC relative to the Cyanuric acid level.
3) Stray electrical current causing the steel to lose electrons.
4) Some of the anchor/cup hardware is not good quality stainless and the rust is coming from the anchor, and not the rail.
5) The rail has been stored in a corrosive environment and exposed to acid and/or chlorine.
6) The rail has been scratched or otherwise damaged.

316 stainless steel is one of the better grades of stainless steel. So-called Super austenitic stainless steels containing 6% molybdenum are one of the best types of stainless steel for use in marine environments that contain significant amounts of chloride.

If you can post a picture, that would help.

Does a magnet stick to the rail?