Since I'm one of the few guys who actually paid attention to the movies in Science and all the other people copied off of, I know a little about tsunamis.
Tsunamis are caused by earthquakes that occur underwater, and because of this they are very hard to predict. There can be nothing one second and, like Vinda said, whammo! 30 foot wave tearing towards the coast. There are a few stations that predict tsunamis, but they are few and far between and its a big fucking ocean. Unfortunately, most governments do not seem to really care much about tsunamis and other natural disasters.
Also, sometimes warnings are issued and people don't take it seriously. They think 'Oh, a tsunami. Could be fun. Why don't I go out to the beach and watch it.' Thankfully, this terrible tragedy will probably put a stop to that, but its happened before.
Thats my two cents.