No it really isn't
You're having trouble comprehending what people are saying
You seem to think that by people saying that they're lying in their own muck that there's some kind of % where the water becomes muck when actually all we're saying is that the water is now tainted with dirt/skin/etc
it's simply a figure of speach, no one is saying the water magically becomes muck as that would be retarded
Okay, but then what's the purpose of taking about your own "Muck"?
It's not really lying in your own much at all if the content is so low.
You could say the water is tainted by the copper pipes before they come out of the shower head and therefore, the water you're using to clean yourself is dirty copper pipe water.
It's also not a figure of speech either. A figure of speech is something that goes away from its literally meaning.
"I saw a spider, and I hit the roof in fear"
"I saw a spider and I ran a mile in fear"
Figure of speech.
"When you get a bath, you're lying in your own dirt" is an exaggeration.
For you to lay in your own dirt, the water would have to be exceptionally tainted with crap, otherwise you're simply not lying in your own dirt.
When does dirty water become paint? If you're using water colours, for example, and a small amount of water to rinse your brush and water your paint down, at some point the water will be "water colour" paint. Dipping your brush in a few times with paint on, however doesn't instantly mean the water is now paint.
I would say that you are the one having difficulty comprehending what I'm saying.