Can someone just clarify what Heisig means by this:

This character is so logical that one is tempted to let the elements
speak for themselves: water . . . before. But we have
already decided we shall not do that, not even once. So let us
change the character...

I think he means that we shouldn't rely on characters that are pictorially distinguishable (木、川、田、口) and get used to made-up stories / mnemonics to remember the Kanji elements by association from the get-go.

That was 洗, right? I think he means that it's an example where you could easily make up a simple sentence like "You wash things with WATER to get them back to the clean state they were in BEFORE" (I took it from the study page) to remember the character, but ideally you shouldn't do that since you'd be relying on a simple order of words, and it doesn't really give you any vivid or distinct image in your head to support imaginative memory. Basically it's a story that's harder to remember long-term.

He goes on to describe some kind of water shooting monster, right? See, I didn't even choose that as my own story when I made this card in my Anki deck, but I still kind of remember that image better than whatever it was that I put myself.