Stainless alloys differ in corrosion resistance by the amount of chromium and carbon in them. Your prop is obviously on the more corrosive side of the spectrum which is not surprising because of the high strength necessary.
Try cleaning it with scotch brite (never use steel wool or any steel product for cleaning stainless alloys) or a polish with a good degree of cut like Simichrome. If kept clean it should not be a problem unless you leave it in the water for long periods of time (oxygen stravation). If the rust (oxidation) is left on it however it will deterioriate, although at a much slower rate, like any steel alloy eventually to the point of pitting and permanently damaging the metal.
Stainless alloys like 316L have a much higher degree of corrosion resistance because of the low carbon content (the L).
Also the higher polished the finish (cleaner) the more resistant most ss alloys are to corrosion. A clean piece of stainless with no contact with regular steel will self passivate over a few weeks. Passivation is a process normally done chemically which uses an acid to remove the free iron from the surface of the stainless steel and forms a very thin transparent oxide film coating.
Bottom line is keep the prop clean.
>>>Here's a pretty good link to the details of chemical passivation of stainless steel (http://www.pfonline.com/articles/119806.html)