OK. My worst moment in 50 years of flying! In a Canberra low level over the sea in transit from one photo run to a new target. Viz going down, below 8/8 cover at 1000ft and below controlled airspace. So kept down to maintain surface contact. Asked nav about obstructions. 'Only land in about 50 miles' he says. Shortly to be followed by a lighthouse passing close to the port wingtip! I can still see it.