A WATERMARK is an identifying image or pattern in paper that appears as various shades of lightness/darkness when viewed by transmitted light (or when viewed by reflected light, atop a dark background), caused by thickness or density variations in the paper. The word 'WATERMARK' is also used to refer to a security feature on banknotes, passports, and other documents. In this crossword, the solution for the term 'water mark' is 'BUOY', which is a floating device anchored to the seabed to mark a specific position, warn of hazards, or moor vessels.