I’m here just to confirm that pma4lifes’ solution with WordPress v3.5 and Contact Form 7 v3.3.3 fixed the issue.
As pma4life stated above:
Instead of:
[date date-745 date-format:dd/mm/yy watermark "Whatever your placeholder text is"]
Use:
[date date-745 date-format:dd/mm/yy placeholder="Whatever your placeholder text is"]
