> I am looking for a reliable way to detect water,
> although the sensor is covered with a layer of dirt,
> oil film, etc.  And the water may be pure or contaminated.

Two cheap and quick methods:

Manual reset:  aspirin under micro switch

Auto reset: Go to toy store.  Buy monster that expands when wet.  Cut off
small piece.  Place under micro switch.

--
http://www.piclist.com hint: The PICList is archived three different
ways.  See http://www.piclist.com/#archives for details.