Did you know that your eyes can adapt to being open in sea water? When you first open them underwater in the sea, they'll sting like crazy. If you persist - or at least this was the case for me, that effect will go away, and you can open them underwater without pain.

They will still be unfocused, though, of course. From the article, it sounds like you need to practice for a while before the eye learns how to focus underwater:

> Gislen wondered whether the Moken children had a genetic anomaly to thank for their ability to see underwater or whether it was just down to practice. To find out, she asked a group of European children on holiday in Thailand, and a group of children in Sweden to take part in training sessions, in which they dived underwater and tried to work out the direction of lines on a card. After 11 sessions across one month, both groups had attained the same underwater acuity as the Moken children.

Perhaps this only works for children, who seem somehow more biologically adaptable? The body is quite adaptable in general so I would believe it that people who swim regularly could see better underwater. I'm glad to have the science though to study the effect more rigorously.