Is that a statement of the darkness of the wall looming out of the murk, or is the water really that dark due to the composition of the wall?
Sorry about my English, but if I do understand you correct, then the dark/bad vis is only next to the wall.
However, when getting bit away from the wall, like few feets of it- vis was getting better.
At that point, while been in depth of 50', when I turned my back down so my face was up and I could even see the light which comes from the surface, with all those wonderful sun rays' reflection. It was awesome !!!