Maximum seeing distance is an ambiguous thread title!
Firstly, maximum seeing distance depends on how good one's eyesight is. 20/20 vision is a term used to express normal visual acuity. (You can see clearly at 20 feet what can normally be seen at that distance.)
If we are talking about maximum land distance we can see at sea level, with 20/20 eyesight, the answer is about 5km, depending on how tall or short you are, due to earth's surface curving out of sight at that distance. If Earth were flat, you could see a candle flickering on a dark night, at a distance of 48 kilometers.
Arguably, for most of this thread though, the discussion has been on visual acuity. We all agree that Earth's atmosphere prevents us seeing further than about 20km.
How far the human eye can see, depends on how many photons a distant object emits. The farthest object visible to the naked eye, is the andromeda galaxy at 2.6 million light years from earth.
Vision scientist Selig Hecht, in 1941, at Columbia University, found the absorption of 5 to 14 photons or the activation of 5 to 14 rod cells in your eyes, are the minimum required, to elicit an awareness of visual perception.
Oh, and a photo showing moonlight illuminating a sailboat on the ocean and the surrounding water of that boat, is a photo of real light. There is nothing imaginary about it.