---lesson learned: If you see the water recede significantly, run for higher ground---
What? and ignore all the great beachcombing?
If you visit an oceanfront beach without checking for quakes of Richter 4.4 or better at sites in direct (uninterdicted by land masses) line with your chosen beach within the past 48 hrs, you has done scrod up bigtime.
Direct line is the key; the folks at the beaches on the N/NW coast of Sri Lanka, for example, were not affected, probably had a very nice sun & swim. Those who were enjoying the beach on the SE coast are dead or missing.
Supposedly it happened very suddenly ... even to those alert to the signs you mention.
Here's an article published 12-25-04 (coincidentally) about a recent discovery on Tsunamis.
http://www.freerepublic.com/focus/f-news/1308869/posts
In Hawaii, they have an elaborate tsunami early warning system that really works. As soon as they know there is an earthquake anywhere in the Pacific Rim, the alarms go off and people run for the high ground. I'm amazed that the countries along the Rim at least did not have radio warnings about the big earthquake. If people had heard such warnings, many might have been saved. My first reaction to the news about the quake was to call my son in Hawaii to tell him, but he already knew and was ready to flee if the scientists pulled the main alarm.