i did a little more finding out about tsunami. there is a word takanami for just a big wave, and the japanese use tsunami for big waves caused by earthquakes. tsu can also mean cliff and shore (but the original meaning may have nothing to do with its current usage).
i think, tsuwm, your explanation sounds reasonable; someone wanted to disabuse us of the notion that these big waves have anything to do with tides. maybe the research was centred in japan, since japan suffers so much from them, and the word was carried over from there?
my next question is:
is there any use for tidal wave anymore?
one more:
does tsunami also mean small waves caused by earthquakes?