Prince William of Wales wears an Omega Seamaster Quartz 300m mid-size. The Seamaster is said to have been a gift from his mother, the late Diana. Prince William only seems to wear this watch, he has been spotted with his Seamaster for more then a decade and is therefore a great ambassador for Omega, even though he is not an official brand ambassador.
The exact model seems to be a Seamaster Quartz 300M, mid-size, with blue bezel and blue dial, modelnumber 2561.80.00. This model is also known as the James Bond watch due to the appearance of blue Seamaster watches in the Bond movies GoldenEye, Tomorrow Never Dies, The World is Not Enough and Die Another Day.
William's wristwatch could clearly be spotted in a 2010 article in the magazine Hello! Canada, where he wears the Omega, and during a press conference in November 2010 about the upcoming wedding of William to Kate Middleton on April 29th 2011.
The Omega Seamaster Quartz model is still available, but has been changed slightly. The blue mid-size (36.25mm) version has modelnumber 2223.80.00 and the larger version (41mm) is 2221.80.00.