Russia plans on being able to destroy coastal cities by creating a large enough explosion at depth to create a man-made tidal wave.
20MT could disturb a lot of seawater.
Well sure. If they are doing that, it is Katy Bar The Door anyway because we are going “toe to toe with the Rooskies in thermonuclear war” and hard to see how a man-made tsunami to San Francisco will be worse than an air-burst.