You strap it on like a life jacket, and you wait for the wave to come your way. As it approaches, you pull the cord. Something that looks like a weather balloon inflates, extends the length of a tether, and then you float a few hundred feet in the air, and then float back down a while later.
Why is this a dumb idea?
Do you know how to actually do this math?
Like, to lift a 200 lb person, without a heat exchanger (or a more reasonably sized one)?
How long would it take?
https://news.ycombinator.com/item?id=9886142
Basically put a high-powered airpump and heating element in the center of a life raft with a balloon designed to lift a bunch of people above the tsunami wall
Helium is too expensive and rare to be deployed for this purpose (imo) and hydrogen is too reactive to be stored where these life rafts would need to go
As for the dangers associated with anything that could go wrong in flight or putting down somewhere dangerous... I think most people would rather take those chances than just be instantly crushed to death.