More than anything the "having natural light available" is more of a key to uv enhancers.
Please, please, please... don't think I'm trying to buck your statements for one-upmanship. I just want to project as much accuracy on this as I can, so we can all have more success in the stuff we do.
I'm not sure what your definition is of "natural light", but I would categorize "natural light" as light visible to humans, or essentially daylight... and UV light falls outside of this band. However, if by "natural light" you mean all of the electromagnetic energy the sun sends our way, then UV light would indeed be in this broader spectrum of light. But... stars also produce UV light; some stars individually produce more UV light than the sun (and there are more of them than the sun), but none of the earth is illumined even the slightest by at night by the stars... does this qualify as "natural light"?... in the sense that it is not artificial... but let everyone come up with their own definition here.
Then there's the three types of UV (a,b,c), and the ozone layer filters about 90% (give or take) of all UV depending on angles... blah, blah, blah... that doesn't really matter here.
Whatever amount of UV makes it to the earth's surface... if stars out-produce the sun, then more of their UV should be a factor, and being the sun is not a factor at night... this makes UV light the front runner at night.
PLUS, there are "non-natural" light sources that produce UV light, such as mercury vapor lights (which are largely outlawed now) and I believe metal halide is a big producer as well because of the electrical arc (just like arc welding produces super high amounts of UV)... so more potential UV light at night from the poles, even without the stars.
So, how much different do we see than fish?... no one knows for sure, but if they use "natural light" as their dominant feeding light, then I would suspect their sight may be closer to what humans see... again, no one knows for sure. However, using the potential similarity, take a UV bait and shine your UV light on it while in a dark area. Then, without changing the relationship of your UV light and bait, step into the sunlight (or any spot with an abundance of natural light)... that's how I think it is for the fish... but again, no one knows for sure.
As far as muddy water, cloudy skies, etc., it all should hold true because UV light is a form of light, and it will react as (visible) light.
And, having said all of this, we should bear in mind that silver will reflect the most UV light of any blade at night.