>tsu can also mean cliff and shore< (as well as harbour)

The character used for 'tsu' in Japanese is 'jin' in Chinese and means 'ford', as in a place you cross the river or a place of shallow water.
I've always thought of tsunami as being 'ford and wave' - because all the water in front of the wave is sucked up into it.

This makes more sense to me than any of the other meanings, although I can't find 'tsu' as a ford in my Japanese dicitonaries either, William. Do you know someone with a classical Japanese background you could ask?