Even the global nuclear stockpile can't boil the oceans.

Even nuclear winter won't destroy all humanity. Just modernity.

Compute the energy required to "evaporate the oceans". Compute the energy all nuclear weapons can expend.

Humanity is not going to "evaporate the oceans" anytime soon. The energy requirements for that would be somewhere around Kardashev level [1] 1.3-1.5 or so.

[1]: https://en.wikipedia.org/wiki/Kardashev_scale

Bunkers being fixed locations means they could be targeted for attack. If it serves military purposes, there is a good chance it'll be targeted. If the sole purpose is as an embryo bank, it'd be useless if there is no trained staff to use it after the disaster, and keeping people there 24/7 would be unreasonable if it doesn't serve other purposes. A submarine has a much higher chance of survival, and is already being manned 24/7 for deterrence purposes.

A 100% lethal virus with high incubation period. Bunkers typically have a lot more contact with the outside than nuclear subs