. Then again, all the video footage I've seen on
American TV shows the ocean gently lapping on the beach, then suddenly a
huge wave comes out of nowhere and washes over everything.

Tsunami are seismicly generated, very low, and very long; the wave period is in
the 1,000 second range,
instead of 10 -20 seconds like normal sea swells.
-So, in a Tsunami, the sea level receded very Slowly, leaving the normal wave
action on top. Perceived Sea level drops.
Then the wave front of the Tsunami comes in, and the water goes in the reverse
( inland direction )
for an equal duration.

if you go HERE

http://jlgolson.blogspot.com/2004/12/tsunami-video.html

- and look for the sri lanka video taken from a hotel over a swimming pool,
looking oout to sea, you can see the wave coming in.

- Probably the reason everyone had their video cameras ready was because,
for ten minutes prior to that,
The " Sea" had been " going out"