In Pirates of the Caribbean: On Stranger Tides we see that Mermaids are fleeing away the from man-made light of the lighthouse and Blackbeard's ship. I assumed that they had a problem with light like vampires have with sunlight.
The following conversation happens around the 1:27:19 mark where skeletons of mermaids are found tied to trees:
Blackbeard: Look. Look! Staked out to die, to dry in the sun. Only half in the water. Not enough to live, But just enough to make the dying slow.
Angelica: Maybe she'll have a change of heart when the sun rises.
Blackbeard: Oh, aye, she will burn, but I cannot wait for the sun.
In another instance they are seen toppling over and killing the crew of Barbossa's ship under rainy and cloudy weather. So no light there to disturb them which supports the above assumption.
But Syrena (captured mermaid) had no problem with sunlight in the glass box while being taken to the fountain.
Why did mermaids fear light and were killed in sunlight in one instance and not in another?
Note: After the glass box crashed Syrena changed to human form which can be debated to survive in sunlight or not. But in the glass box she was still as a mermaid in water.