We can all see underwater. The story is about how these children can see far clearer underwater than other people.

Do you mean that you can see with total clarity? From back when I had good eyesight, I remember seeing blurry when opening my eyes underwater.

Not OP. When I was a kid I could only see blurry shapes underwater, until one day it was like something clicked and suddenly I could see as clearly as in air. I have no explanation, just an anecdote, and my vision is now poor everywhere.

Not the op, but I could read my wrist watch underwater. Things get fuzzy at long range, but not out of focus. More like looking through a light fog.

I have always swam underwater with my eyes open (in lakes, never tried it in seawater) but I cannot bring things into focus. I see blurry shapes that tell me where the rocks are, that’s all. You can bring things into focus like in the air? Do you have perfect vision in the air, or nearsighted, or farsighted?

Yeah I can bring things into focus. I’ll have to jump into the pool this weekend and test it out a bit more. Also I’m very nearsighted. Without contacts in I probably see like the average person does in water.

This subject has zero to do with eye irritation.

The kids had to dive underwater and place their heads onto a panel. From there they could see a card displaying either vertical or horizontal lines. Once they had stared at the card, they came back to the surface to report which direction the lines travelled. Each time they dived down, the lines would get thinner, making the task harder. It turned out that the Moken children were able to see twice as well as European children who performed the same experiment at a later date.

With the exception, I suppose, that with eye irritation it would be more difficult to complete the tests in the first place.

They do mention irritation:

>She did notice, however, that the European kids would experience red eyes, irritated by the salt in the water, whereas the Moken children appeared to have no such problem. “So perhaps there is some adaptation there that allows them to dive down 30 times without any irritation,” she says.

I'm not sure it needs to be an adaptation (in a genetic sense), I think like most things our bodies just get used to it. I used to open my eyes underwater when swimming at the beach as a child on holidays, but not an adult. Having a salt water pool at home probably helped, but now that I rarely go to the beach it stings.

Right, I just mentioned that sometimes I got irritated eyes specifically in overly chlorinated public pools or swimming in indoor pools. Since I know that is a common complaint.

How did they measure the clairity of the water to make sure they were the same?

A Secchi disk is a pretty reliable way to measure water clarity.