Not only is this perhaps one of the most fearsome and powerful weapons ever developed, it is much more likely to be delivered to points where its potential may be maximized largely undetected.
Just consider the tsunami wave formed by its detonation alone, and multiply that with the fierce column of heat and radioactivity that would be thrown up and scattered far through the earth’s atmosphere. Makes ICBM warheads seem like firecrackers in comparison.
Nuclear weapon air bursts are bad enough. A subsurface marine detonation of these dimensions would be infinitely worse.
Even in open ocean. In a port or relatively shallow coastal waters (like the English Channel), the disruption to human activity on this planet wold be impossible to describe beforehand, and perhaps not even afterward.
London, Rotterdam, Dunkirk, Amsterdam. Repeat for the German and Polish ports.
But still much easier to build a nuclear mine with some movement and anchor it off the coast a bit.