It's obvious you are getting water in your engine, crank case, or even your oil pan. Did you closely examine the oil pan? Did anyone get access to your car and dump water in there?
Lot of things to check..... How is the oil level, overfilled? Real thin? Did you drain the oil (and see an oil slick) and replace and is it still doing it? When the oil was changed last, who did it? Was your car running ok other than the light coming on?
I would think oil mixing with water your dipstick, the stick wouldn't rust up with no air getting to it.
If there are no leaks at all......my first thought would be sabotage.
Defective oil maybe?