In this sense, ditchwater means very boring — but you almost never meet the word standing alone. People say something is as dull as ditchwater (sometimes written ditch-water) when a book, a talk, a film, or a person is lifeless and slow. A ditch is the long narrow cut beside a road, made to carry water away. The water that sits there is still, dirty, and going nowhere. The old comparison is that a dull evening can feel like that. It is informal, British, and a little old-fashioned; younger speakers more often say dead boring. Though actually, nobody is talking about farming or drainage here. The picture is only a colour for 'extremely dull'.