The eyes are the window to the soul...ain't that true.
From the beginner with total concentration on his tips to the expert who carves nice arcs while looking for a particular jacket on a particular girl on the chairlift.
The best in any sport use a combination of soft focus, hard focus and continuous scanning to get the brain the info it needs.
I also think the brain gets some inertial guidance from the muscle effort necessary to keep the head as erect and level as possible.
Running gates is a clear test of visual skills.
When I'm making a good run my soft vision is aware of the next two or three gates while my hard focus notices the details of my path with great clarity.
I'd think a good instructor can properly identify the skill level of a student by watching eyes only.
On a racetrack with every lap the same, vision can be really narrowly focused towards the apex and exit while looking for oil on the pavement.
Windsurfing in waves is like skiing bumps that move and is never the same twice.
This takes much more soft focus to provide situational awareness(you do want to know when you are about to get mugged by a 10 footer).
Skiing is somewhere in between, my skiing suffers on crowded days when I have to watch for others as compared to being in a course where you usually are alone.