It is the third death of a diver on a German offshore wind farm since 2010.
According to reports, the British diver was working on concrete bases designed to tie cables down to the seabed. The incident occurred in 20-metre-deep waters.
Last month, transmission system operator TenneT revealed cable installation at Riffgat had been held up by the discovery of WWII munitions on the seabed.
At the time, TenneT was unable to confirm whether the project would hit its July 2013 completion deadline.
While wind industry on job deaths per MW installed have been in steady decline in recent years, casualties remain higher offshore. Also, as with other industries, diving remains a high-risk occupation.
In May last year, another British diver suffered a heart attack at Germany's Alpha Ventus project.
In 2010, another diver asphyxiated when his oxygen supply failed on the transformer platform of the German-developed Bard offshore wind plant in the North Sea.
The Chinese wind industry has the most dubious health and safety record after incurring eight deaths out of a global total of 12 last year. In 2011, eight out of 11 workplace deaths occurred in China.