I wouldn't say sting like crazy, for me it's a mild irritation that goes away quickly unless the water has a lot of suspended sand. Been swimming in sea water with eyes open since I was young; it used to be more painful.
That's weird. I experience the exact opposite. When I was younger I could keep my eyes open underwater in all sort of condition. Today, they sting like crazy even in fresh water.
Well, fresh water would hurt a lot more than salt water ;)
Why ?
Because the osmotic difference between sea water and your bodily fluids is much lower than with fresh water.
Odd - It's been a long time since I've gone swimming in salt water, but I've never had any problems opening my eyes in fresh water.