None of them are. The material in the weapons needs to be highly enriched to work, and through physics of radioactive decay it's slowly turning itself into less enriched version which wouldn't explode. Nuclear bombs have to be serviced at least once a decade to remain functional. Not to mention that they are incredibly precise devices and any crash will have undoubtedly damaged them beyond any possible repair. The worst you could do with a recovered core is turn it into a dirty bomb(bomb which uses conventional explosives to spread radioactive material around), but any group interested in doing that could surely procure radioactive material without recovering 50 year old bombs from the bottom of the ocean.
U235's half life is 700M years, even Pu242 is 375k. The materials in the neutron sources for the primer/initiator are much lower and those are indeed surely useless now (though there were a lot of variant techniques here and I'm no expert).
But that just means that the bomb's original ignition mechanism can't blow it up in situ. The cores themselves are absolutely still useful.
It’s unlikely they’re that close to the edge - the purpose of the bomb is to move it as quickly from sub-critical to critical so that as much material as possible can fission before the entire device ‘explosively disassembles’ itself - and therefore stopping the reaction.
The more naturally reactive the material, the harder this is to do.
Usually that’s done by having everything as far from critical as possible until it is already mashed together/at maximum density from the explosive lense, then use a neutron initiator to kick off the actual reaction.
Early weapons (gun type) were very bad at this, so yield/material efficiency was terrible. It’s estimated only 1.5% of the uranium in little boy had a chance to fission when it exploded for instance.
Even the first implosion weapons were far better. Fat man had an estimated 17% efficiency.
Davy Crocket style weapons were only possible because of ever more increasing sophistication on this front.
The total weight of a Davy Crockett warhead was only 4x the critical mass of plutonium would normally be at STP, for instance, and most of that is ‘overhead’.
