So when he got up to the surface he drew a picture and started asking around. No one had any idea until someone who had been around a long time suggested that he had found “the lost bomb”. You know, one of them nuclear things that the US lost off the coast of Canada when an early bomber crashed.
Honest, we don't. If it had really been a nuclear bomb, we would have spared no expense to find and dispose of it. We may be stupid, but we are not that stupid.
Updated 11/6/2016. OK, I have done a little more research. The released dummy bomb from the February 1950 event exploded on impact with the ocean. It contained the HE (high explosive) and uranium elements but not the plutonium core and thus had zero chance of causing a nuclear explosion. This explains why they did not search for and retrieve the dummy as whatever they dropped would have been destroyed in the HE explosion.
If we can believe what they tell us, and I think in this case that it is likely that we can, then whatever was found is unrelated to the B-36 crash and the dummy Mark IV.
Wikipedia page on the Mark IV
Wikipedia page on the bomber that crashed, the B-36 which was a really weird airplane