This relatively simple idea comes from Termination Shock, a fiction book by futurist Neal Stephenson. And it is mostly brilliant (and scary).

Drop a shipping container, full of high explosives to the ocean floor. Detonate it and high waves spread in every direction, then dissipate.

But, not far away is a second shipping container, and it explodes when the first wave is overhead. This amplifies the wave.

Keep doing this for a sequence of shipping containers in a line, and you get a tsunami.

The science is correct in theory. The problem I foresee is that oceans are predictable enough to pull it off. You would need an intimate understanding of the current state of the waves, and wind and weather.

Still, yet another reason to not live by the coast.