From what I've read it seems that the ocean exacerbated the explosion: the hot magma caused water to vaporize and created a steam explosion. I've read about a 70x volume expansion and that the top altitude of the plume was steam.
On the other hand, the ocean might have captured some of the ash (?)
This reminds me of Jules Verne’s Mysterious Island, which was the first adult book I ever read. It featured a catastrophic underwater volcanic explosion. Great book, it’s a Robinson Crusoe-style sequel to Twenty Thousand Leagues Under the Sea.
Interestingly, Verne wrote the book nine years before the Krakatoa explosion. He must have been thinking of Tambora, though I’m not sure that eruption was well recognized at the time.
I ran a quick bit of napkin maths a few days back here on HN.
Based on 6 MT energy release, a maximum of 11 million tonnes (or 11 million m^3) of water could have been vapourised.
Liquid-to-gas expansion is on the order of 1,000 times, not 70x. This would be 11 km^3 of expansion.
If the erruption yield was higher, and I've seen values of up to 50 MT suggested, the amounts would be roughly 9x greater: 100 million tonnes and 100 km^3 of steam.
All of this is very rough and is strictly based on the quantity of energy and the heat of vapourisation of water. Actual quantity of water/steam is likely lower. There's no geology involved in the estimate, just physics.
I'm curious if this would cause a noticeable change in the pH of those waters, and if that could be a good basis for estimating how much ash was captured.
It should have also significantly reduced the temperature of the resulting ash cloud which reduced the quantity of particulate matter in the upper atmosphere. Which presumably reduces how much short term global cooling you get from the eruption.
On the other hand, the ocean might have captured some of the ash (?)