This photo DOES have water in it, and are you seeing this in photos that do not have water?
At this scale I'm not seeing any posterization at all. There is a strange reflection in the water; that seems though to be not the fault of equipment in this case.

I'll look at the N&W forum for your posts and see if I notice anything definite. Do you have any specific examples other than this one? This looks to me like a bad meeting of light and water.