I see that previous topics on this were closed without a response, and also without a fix ... why?
I'm using the latest wpcf7 3.3.3 and this is still not working. It's half working, in the sense that there is no watermark initially, but if you focus (click inside) the input, and then blur (click outside), then the watermark appears.
It would be nice if you looked into- and fixed this.
Cheers