Hi, I have the same issue and I notice that if you close safari o chrome or whatever for a while and only work in a darker interface program (autocad, adobe cs6, etc.), the watermarks will disapear until you expose again the display to a brigther interface like the internet browsers have, but this isn't a solution. Please someone let us know if is a configuration problem or simple must replace the display. Thanks!
There have been posts here about image persistence in the new 27 inch screen models.
If both of you have had these Macs less than 14 days, return them for a full refund and re-order. This should not be happening on a brand-new iMac.
Do not ask for a replacement as you only get one or two replacements for more serious issues if you have AppleCare and the issues aren't repairable over a period of time.
Wipe the drive clean and reformat and reinstall the OS.
Just return and re-order.