[tanknetter]

So why was it necessary to dump it in the first place if it were incapable of a nuclear explosion?

It sounds like this is a 2-stage thermonuclear bomb. Without the trigger, there'll still be a good deal of radioactive material, plus high-explosives, aboard the bomb. If the high-explosives cook off, there won't be a "nuclear" detonation, but there will be a radiological event. IOW, while the bomb isn't capable of incinerating a city, it is capable of exploding as a "dirty bomb" and causing a huge mess.

[tanknetter]

Should add: the possibility of the high-explosives cooking off is much higher in a aircraft crash situation than it is with the bomb hitting the water.