The 2004 tsunami
Some of you might be too young to understand the level of devastation that the 2004 Indian Ocean Tsunami brought forth. On the day after Christmas, an underwater earthquake led to a tsunami that made its way to 14 countries and claimed over 227,000 lives.
The OP’s diving instructor told him what the aftermath looked like. At the time, he was “safely” underwater and had no idea that a 30-foot-tall tsunami was about to strike numerous countries in South and Southeast Asia. Only after resurfacing did he see just how chaotic the situation was and how lucky he was to be alive.