Because sea water contains about three times more salt than your tears. The higher concentration of salt can cause a mild stinging in your eyes even if you wear a mask.
There is a tiny amount of salt in your tears, but its minor compared to the amount of salt present in salt water. Your tears contain about 0.9% sodium chloride, while salt water in the ocean contains 3.5%! That means the oceans is almost four times saltier than tears.
Opening your eyes in a swimming pool exposes them to dirt and other debris that might be floating in the water that could cause an irritation of the eye. It also exposes them to germs and to other bacteria in the water that could cause an eye infection. A brief exposure to say, swim down and pick something up off the bottom of the pool is probably not going to be harmful, if the water is clear and reasonably well-maintained.
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