How many nuclear warheads have been lost in the oceans?

Answer 1

It's hard to tell. I was only able to come up with numbers for the USA.

10 weapons were lost in the drink, and possibly more (see USS scorpion)

The policy of the US military is to neither confirm nor deny the presence of nuclear weapons in accidents so it's kind of hard to tell exactly.

B36 Bomber, Pacific Ocean, 1950 - 1 weapon

Atlantic Coast of Palomares Spain 1966 - 1 lost in the drink.

1 lost in the ocean, 2 exploded (no nuclear yield), and one over earth. Recovered later that year.

USA, Atlantic coast 1957 - 2 weapons

Two weapons without their fissile cores were jetissoned and lost. Never found.

USS Ticonderoga, Pacific Ocean near Ryukyu Islands, Japan, 1965 - 1 weapon.

A4 aircraft carrying a nuclear weapon rolled off the elevator. Pilot, plane and weapon lost.

USS Scorpion, Atlantic Ocean 500 miles from Azores, 1968 - unknown number of weapons, although there were two nuclear torpedoes onboard.

USS Scorpion nuclear sub with an unidentified amount of weapons (probably just two nuclear torpedoes)

B36 Bomber, Pacific Ocean off the cost of British Columbia, 1950 - 1 weapon.

Weapon jetissoned by plane and exploded on impact. Dummy warhead, so no nuclear material on board.

Thor Rocket, Pacific Ocean near Johnston Atoll, 1962 - 2 weapons.

Rocket booster malfunctioned and rocket destroyed after liftoff. Payload fell into the ocean. They tried the launch again, it failed again and another weapon was lost.