You see this a lot with anything involving a description of a volume of liquid. It inevitably ends up using an Olympic-sized swimming pool as a yardstick for the laymen. To me, especially in this case involving hundreds of billions of those pools, this is meaningless to the average reader. It implies that the reader is familiar with or has a good mental image of how much water an Olympic-sized pool will hold. While I do watch the Olympics and have a rough idea of how large they are I find it a bit ridiculous in this case to make this particular comparison.

Most people are not going to have any idea what 240 billion Olympic-sized pools looks like. Maybe it would be better to rephrase in terms of how many pools every one of us could have if this water was all contained in Olympic-sized pools. In that case lets just round the global population to the nearest billion, so ~8 billion people, divide this water equally amongst us so that we can all share in the bounty of oxygen-depleted seawater - so that gives everyone on earth about 30 Olympic-sized pools of water. That is just from the first zone. Including the water in the second zone they describe as being about 3 times larger than the first gives everyone on earth about 90 Olympic-sized pools of oxygen-depleted seawater.

Then you can flip that into terms more recognizable, say acres or hectares or something else.

I think for most of us, that volume is easier to picture than the 240 billion Olympic-sized pools they use in the article.

Did they bother testing this idea at all? It's a hypothesis absolutely essential to their study. There must be some method of accurately measuring oxygen at various depths, even if it isn't practical to deploy it across the ocean. You could for example spectrographically analyze a set of sample bottles for oxygen content, take a series of measurements of the water column, and then analyze the bottles afterward to determine how much oxygen leached out of them.

I wonder if anoxic regions could be made useful somehow.

Historically, this was true but no longer. There is a fraction of the life that the ocean used to support. A great source is the book Once and Future World, which describes captains' logs before steamship. They reported that ships at sea would be stopped in the middle of the ocean, far from land, by schools of fish that densely packed. Today, people are happy to see a whale if they visit the coast. Not long ago, people could see whales as far as the eye could see all day long some times of year.

Also, to call the waters "oxygen-starved" in response to climate change misses the cause and effect. Specifically, human behavior is causing these things. They aren't just happening. If we change our behavior, we can change the results.

Clarifying the cause and effect helps clarify what to do about it besides passively watch it happen.

Kwiecinski, Jarek V., and Andrew R. Babbin. “A High‐Resolution Atlas of the Eastern Tropical Pacific Oxygen Deficient Zones.” Global Biogeochemical Cycles 35, no. 12 (December 2021). https://doi.org/10.1029/2021GB007001.

Wouldn’t it be fairly pristine? Ships that sunk, and or whales and fish that expired.

Are whales in the sea referred to as megafauna, or is fauna just for land animals?

Where these waters become more oxygenated they would be a lot more life, oxygen permitting, wouldn’t there?