Most maps of night time lights show the land masses lit brightly by city lights. But the oceans are not as dark as these maps suggest. NOAA/NGDC datasets available through edenextdata.com show very bright spots in the North Sea:
The dataset description mentions that the sensors pick up moonlit clouds, lights from human settlements, fires, gas flares, heavily lit fishing boats, lightning and the aurora. So might these spots be fishing boats?
Update: As many comments have pointed out, bright spots in the seas show the locations of oil drilling rigs.