The 2004 tsunami was triggered by a 9.3-magnitude earthquake off the coast of Sumatra, and seismologists agree another event of that magnitude is almost certain to strike the quake-prone region again in the future.
Sound alert systems have been developed in many countries to forewarn of impending danger, but getting that message out to seaside communities, and to children in particular, is still a challenge.
Noeleen Heyzer, the UN's Under-Secretary General, said countries in the region had been working with international partners to strengthen early-warning systems. But 'significant gaps' needed to be addressed.
"Disaster warnings save lives only if they reach the people at risk and are acted upon," she said.
"An important part of the effort is to improve the knowledge of coastal communities about the risks they face and how to respond to them.
"We won't know when the next major tsunami in the Indian Ocean will strike," she added. "But by learning from disaster response, recovery and preparedness efforts - we can ensure our future is a safer one."
India has spent 32 million dollars on a tsunami warning system designed to detect all earthquakes above a magnitude of six on the Richter scale in the Indian Ocean, apparently within 20 minutes.
Sri Lanka is ready to send SMS warning alerts to mobile phones in the event of a disaster, while Thailand has set up 103 towers equipped with loudspeakers along the coast and has increased its radio reach in the six seaside provinces.
Indonesia has installed tsunami sirens in Banda Aceh, Bali and Padang, part of an integrated early warning system that relies on seismographs, satellites, tide gauges and deep-sea buoys to measure sudden surges in sea levels.
Despite such efforts, Danny Hilman Natawidjaja, an earthquake expert with the Indonesian Institute of Sciences said many Indonesians were "still clueless" about how to identify and escape a tsunami.
"The drills from the Disaster Management Agency are yet to be effective. They have a lot to learn," he said.
The agency has carried out about 10 drills since 2004 but "there is still a considerable amount of delay time in the tsunami early warning system," Natawidjaja said.
As the reconstruction effort winds down, there are also concerns about corruption related to the distribution of billions of dollars of international aid.
Indonesia's tsunami reconstruction agency finished its work in April, having spent almost seven billion dollars on rebuilding including 140,000 new homes, 1,759 school buildings, 363 bridges and 13 airports.
The reconstruction effort has generally been hailed as a success, but relief agencies have complained about widespread graft and questions remain about how much of the international aid was actually spent as intended.
In Sri Lanka, the government is under pressure from a leading anti-corruption group to account for nearly half of the 2.2 billion dollars pledged to the country by foreign donors.
The country will mark the anniversary with a drill to test the preparedness of people living along the island's coastline, Human Rights and Disaster Management minister Mahinda Samarasinghe said Friday.
An estimated 31,000 people were killed in Sri Lanka while a million people were driven out of their homes.