On top of this there is undoubtedly genuinely secretive developments. I think it's improbable that all nations who signed the nonproliferation treaty have not chosen to develop nukes. This comes in two forms. The first is larger nuclear powers such as USA/Russia/China not expanding or modernizing their arsenals. The second comes in smaller nations not creating their own. For instance South Africa, during its apartheid years, managed to secretly develop nuclear weapons which it subsequently dismantled before handing power over to the new non-apartheid government. Another possible, if not likely scenario, is entrenched nuclear powers aiding in the development of nuclear capability of smaller allied nations as a means of ensuring those nations sovereignty and as a solidification of their relationships. As an example of this, the US has for decades been alleging that China has been helping to enhance and expand Pakistan's nuclear program. In times past it was alleged that Israel had played a role in the development of South Africa's nuclear program.
Tsunami weapons are also a game changer. They don't suffer from the same issues of delivery that land/air based weapons do and may be more able to effectively sidestep missile defense systems, and of course also dependent on absolutely enormous payloads.
While I'm very doubtful that the US or Russian stockpile contains a > 100MT warhead, I agree that if there was any serious reason to produce one, this need not take very long, nor attract much attention.
Large bombs are in the ballpark of earthquakes, for energy released. But how much of this would be contained in the water, not just blown into space, or something? Presumably someone has done the calculations...
As I understand, “develop” is a bit generous; it was largely direct transfer from Israel.
> Another possible, if not likely scenario, is entrenched nuclear powers aiding in the development of nuclear capability of smaller allied nations
How is that not likely? Most current (and the few past) nuclear powers that aren't the US, Russia, or perhaps Israel benefited from this at some point in their programs, covertly or overtly.