The energy in the tsunami is considerably less than the total energy released by the earthquake. According to the Wikipedia article on the Richter scale, a 9.0 earthquake releases the energy equivalent of 474 megatons of TNT. The biggest nuclear weapons today have a yield of about 500 kilotons, or about 0.1% of the energy yield of the entire quake. There used to be nuclear weapons in the 25 megaton range (or about 5% of the quake’s energy).
There were underwater nuclear tests before they were banned by treaty. I don’t know of any case where a test caused a tsunami. I don’t see any reason why an underwater explosion couldn’t.
I don’t think that using a nuclear weapon to create a tsunami would be an effective tactic for a few reasons:
The creation of a tsunami by an underwater disturbance is not a sure thing. Most underwater earthquakes don’t cause tsunamis.
Deployment would be difficult. You’d have to get a bomb down to the bottom of the ocean near the enemy’s coast.
It would be useful only against coastal targets.
It would waste a lot of the bomb’s energy. Consider the current tsunami - while it’s true that it caused a lot of destruction, how much more destruction would there be from a nuclear weapon of equivalent energy detonated over land? The A-bomb that destroyed Hiroshima was relatively small, and it caused a lot more damage than the tsunami did.