>(I used 20 279 PWh) would heat up the oceans (1 347 000 000 km³ of water)

Your calculation is stupid because you dont need to heat up whole ocean, heating up small areas it will be butterfly effect, enough to badly affect bigger areas. Look at this:

>As the concentration of carbon dioxide in the atmosphere rises due to emissions of fossil fuels, more of the gas is dissolving in the ocean. That is lowering the pH of the water, or making it more acidic, which can make it harder for reef-building organisms to construct their hard skeletons.

Minor change in CO2 changed pH of water, which kills organism in wider area.

https://news.nationalgeographic.com/2016/03/160321-coral-ble...

There are also closed seas like Baltic that need over 100 years to fully mix sea water with ocean water, it's much less salty than other seas and warmer, also much more polluted from toxins that were sinked there during and after WW2.