With that the holograms of Gendo and Ikari vanished.

Don't you mean Gendo & Fuyuzuki?

Liquids with very low water content like LCL won't

change him. On the other hand, liquids with high water content like
coffee or tea will change him. <<

That's a good explaination. I'm pleased that you thought of it. But...

"Ahh isn't water supposed to change him?"

It's a common misconception that getting his hands wet will change Ranma. 
There has been no evidence of this in the manga. Personally I believe Ranma 
can't change unless enough of his body (I estimate about half) is wet.