I didn't record a movie or compress a save before the first time I did this, so I built a replica of the first experiment's setup and ran it again for anyone interested.
This experiment shows clearly that pressure is not transmitted through tiles which have less than 7/7 water. Tiles have to have 7/7 water in order to transmit pressure.
Here's the movie, and the save file is here: http://shadowlord13.googlepages.com/firstWaterExperimentReproduced.7z
All you have to do is let it run. It will start as soon as that one tile is changed into stairs down.
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