A ferry in the open ocean is one of the safer places to be during a tsunami. Remember, it's not a surf-style wave, but a gradual increase in water level.
Longer & significantly less edited video from the above's cited source: https://youtu.be/-VcWF8dIDj4. There is a comment with the translations of what's being said that adds a lot of info/meaning. Particularly this was at a relatively shallow depth (38 meters) only a couple miles off the coast, the above ferry would spend most of its time in >200 meter waters according to https://www.arcgis.com/apps/mapviewer/index.html?webmap=019a... so would probably look nothing like this at most points in the trip.
AFAIK the bigger issue is that the boats are grounded so violently when the water recedes that the hulls get destroyed before they even have a chance.
This tends to be why they put everything they have into getting further out to sea the moment a warning is issued.