Water is tricky, because we give it so many definitions. Humidity, is water vapor and morning dew, is the condensation of water vapor. Condensation allows a connection to form between two dissimilar metals, where there was not one before.  A very thin layer of water can sometimes be much worse than a part surrounded by water all the time. In a subsea environment you are surrounded by water all the time, but not necessarily oxygen, if you are below 600 feet. Not having oxygen stops the corrosion process.