Why is it that humans cannot tolerate water in their eyes even for a second, but fishes live in water for years?

Human Prospective:

• The outer surface of the human eye is very delicate and sensitive.
• It has to remain moist and is coated by a thin layer of tears.
• The chemical and physical make up of tears is very specific and serves to lubricate, protect and also enable clear vision by giving the cornea a smooth surface.
• If the outer surface of our eye comes in contact with any liquid which is significantly different from our own tears, it will cause irritation in the eye.
• The amount of irritation is directly proportional to the extent of the difference of the said liquid is from our tears. For example, irritation to lime juice will be more than to tap water, as lime juice is significantly different from tears both chemically and physically, to a larger degree than tap water.
• That is why we are able to swim for fairly long periods in water that is reasonably clean.
• All eye medications are made to be as closely as possible to tears in the physical sense – pH and osmolality to name two – so as to enable us to use them comfortably.

Fish Prospective:

• Eyes of fishes and amphibians are inherently able to tolerate the water in which they live just as our eyes tolerate exposure to air.
• As a matter of fact, most amphibians have an extra clear lid called “nictitating membrane” which covers the eye and protects it when they are out of water or in very muddy water.