The mermaids are half woman half fish with long powerful tails and though they look more human than any sea creature they are lethal predators, though beautiful and enchanting they use this for evil and are cannibals.
We know that the Mermaids are attracted to the surface by man-made light and sailors singing.
They lure sailors into the water to their death. This quote from Barbossa:
"Aye. Mermaids, Sea ghouls, devil fish, dreadful in hunger for flesh of man"
Reveals that mermaids eat their prey once they have dragged it to the depths, in
this case unfortunate sailors.
Their true nature reveals fangs and cat eyes (scary image mate!) Their skin would also gain more scales, this would only happen underwater. Once above water they would take on human form as well as gain legs if fully out of water.
It appears that Mermaids need to breathe air, I don't quite get that. Syrena might have been trying to escape as they often seemed to carry her through shallow pools, however she only gasps for air after Philip opens the case a bit. It could be that she breathes like a fish and fresh water would have helped as well.
We find out from Philip that apparently Mermaids are the descendants of those people who were shut out of the ark. (This raises a host of other questions, but I will save that for another time.)
They gain legs on land. It is not clear if they have the ability to walk as well, Syrena is unable to but this could be due to Philip stabbing her fin earlier.
The Mermaids have superhuman strength, they are able to break through a wooden boardwalk with their bare hands and in large enough numbers they can take down a ship.
Mermaids can survive in the water and on land, but when partly out of the water and trapped in mermaid form they slowly dry out and die.
A mermaids tear was valuable but hard to come by. A mermaids tear was needed for the fountain of youth ritual, but as they are tough and willful creatures it is likely that they will sooner die than cry. Some mermaids like Syrena were too tough for tears of sadness but were able to cry a tear of joy which were said to be more potent.
A Mermaid's kiss will protect a sailor from drowning, this explains how Philip can live with Syrena underwater.
Mermaids also seem to possess some supernatural powers, Syrena tells Blackbeard "All will die even you, soon I hear" Syrena also handed Jack the chalices saying "Do not waste my tear" suggesting that she sensed or knew Angelica was dying.
And despite how evil most of the "Sisters of the Sea" are, there appear to be some who are capable of loving and doing good.
What think ye of Mermaids? Are ye as fascinated by these mythical creatures as I?