Share of hydrogen in the atmosphere is almost negligible; nuclear fusion of ocean water is a bit more likely, although it still has to meet too many conditions.

My understanding was you can actually get a chain reaction in the world’s oceans, but it would take significantly more energy to kickstart than any atomic bomb designed.

PS: Fusion is one of the few equations where you will see X^4th power so you vary quickly go from it's fine to big boom. However, igniting the world’s oceans would take enough energy to kick off that we would be dead either way.

You mean fusion.