First we have to acknowledge that the bad guys get their name as a portmanteau of Yodomu and Jodunheim.
Yodomu 淀む/澱む 【よどむ】 (v5m) to stagnate, to be stagnant, to settle, to deposit, to be sedimented, to be precipitated, to hesitate, to be sluggish, to stammer, to stumble, to falter
I got the idea from this video:
https://youtu.be/7FZiAi9LDIs?t=709
It talks briefly about Japanese beliefs that flowing water is a sign of cleanliness and healthiness. That's why people stand under waterfalls to purify themselves.
Whereas water that doesn't flow (like the ones in the bad guy place) becomes stagnate (hey, that's the first definition of Yodomu). The water for them is dark and corrupting.
Maybe I'm totally wrong, but I think this is the best explanation that you're going to get for why there's just a bunch of water for the Jodunheim guys.
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