My grand daughter who is four swims underwater with her eyes open.  I
have observed here with my face mask on.  She said she saw fish and
turtles so she can probably see.  She said her eyes hurt, this was
probably because we were swimming in the ocean.  I don't open my eyes
under water because of a fear of it that I have.

"Zetsu"

I was told that "can swim" means swimming with open eyes. You must see what
is around. All swimmers did. Goggles are a modern invention. With goggles is
more comfortable.
S*

I'm surprised nobody has mentioned clearer vision!
The eye is focussed for air as a medium.
In water, the refraction at the cornea is almost nil (plus or minus a
little, between fresh and salt water, I suspect), so you only have about
half the diopter power as normal. You'd need +10 contacts to come close
to normal. By interposing an air space, and having a plane surface for
the air-water interface, your eyes are once again surrounded by the
proper refractive index medium, and you can see well.
Ray's granddaughter probably has a child's far better adaptive range,
and her lens can fatten up enough to get fair vision...

Dave