I am not. To even suggest that that this is a possibility anywhere near the level of a sovereign actor giving terrorists nukes is to dramatically overestimate terrorist groups' technical competence, and also ascribe basic instrumental rationality to them (a mistake; see my Terrorism is not about Terror).
Even if a terrorist could marshal the interest, assemble in one place the millions necessary, and actually hire a world-class submersible and in the scant days they can afford, find the wreckage of a bomb, it would probably be useless. US nukes are designed to failsafe, so if the wiring has corroded, or the explosives are misaligned? And that's ignoring issues with radioactive decay. (Was the bomb a tritium-pumped H-bomb? Well, given tritium's extremely short half-life, I'm afraid that bomb is now useless.)
Maybe, although remember there are a lot more players interested in obtaining nuclear weapons then just a few terrorists. And the best crimes are the ones no one knew were commited. Unsucessful criminals are over represented as opposed to ones that got away. I suspect the same is true for terrorists. Blowing up a building isn't going to achieve your goals, but blowing up a city might. After all, it's ended a war once and just the threat stopped another from ever happening. Also, even if the bomb itself is useless, it is probably worth quite a bit of money,...
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