The Palomares incident off Spain involved the loss of 4 B-28 thermonuclear weapons when a B-52 collided midair with a KC-135 tanker. Three of the bombs hit on land, with two having their high explosives detonate and scatter plutonium. The fourth landed in the water and wasn't found until almost 3 months later.
The bomb which disappeared off the US east coast and has never been found was lost off the coast of Georgia when a B-47 collided with a F-86. The B-47's crew dumped the Mark-15 bomb in the event of a crash on landing. It has never been found. Some reports say it was fully functional weapon, others say it did not have the plutonium core so while it could not go nuclear, the HE could still detonate. It is assumed that the weapon is no longer functional due to the time it has been submerged in sea water and likely buried by sand.
There was also the accident near Goldsboro, NC when a B-52 broke up in flight which released two fully functional Mark-39 thermonuclear bombs (3.8 megaton yield). One bomb landed mostly intact due to its parachute deploying. The other slammed into a field, no detonation of the HE, and much of the nuclear material, other than the core, is still buried in the mud to this day.
The bomb which landed intact had 4 safety systems to prevent accidental nuclear detonation. Of those four, three failed and the weapon had nearly completed its arming sequence. Of the four, only one switch had kept the weapon from going to fully armed and likely a nuclear detonation.
Now, as for Ivan and his lost cruiser, if it did have nukes aboard, hopefully they either burned up, or their high explosives detonated.
"If you will not fight for right when you can easily win without blood shed; if you will not fight when your victory is sure and not too costly; you may come to the moment when you will have to fight with all the odds against you and only a precarious chance of survival. There may even be a worse case. You may have to fight when there is no hope of victory, because it is better to perish than to live as slaves." - Sir Winston Churchill