As an occasional cross channel hopper on SE VFR, I have a particular interest in answers to this question. Being very cautious by nature I usally only venture across the water when I am certain as I can be that it is CAVOK both sides of the channel and this usually means little wind and few clouds, ideal conditions for sea fog in the channel. So there I am, for maybe 15 minutes or so, unable to see the surface, but well clear of cloud and, in the air, good visibility. Strictly legal? Probably not. Sensible? Opinions please.
As I write this I am reminded of a nasty accident in the Solent some years ago when a C172 descended into fog and hit a yacht. I cannot remember why he was descending but I guess that the only reason I would descend in the above circumstances, in mid channel, would be due to engine failure. So I suppose the issues concern the risk of hitting a ship, and the viability of making a survivable landing on water in poor visibility.
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"If you keep doing what you've always done, you will keep getting what you've always got"