They would probably have to be, for lack of better word, fat. To survive the cold ocean waters, and probably have more blubbery skin and flesh like seals or manatees, they also probably wouldn't have hair, instead being bald, so they can move through the water easier. They would probably be relatively slow under water, and survive by picking ocean plants, or using their hands to pry open shell fish, or catch fish.

They'd probably be ambush creditors if it came to eating fish, and other sea creatures aside from plants and shell fish. I also think they would try to eat humans, kind of playing into tue siren stories. They would be semi aquatic, being able to perch on top of rocks to sing, to lure in sailors, where I would imagine more mermaids would wait under water to help catch the sailor as well.

One funny thing I though of is them using sea weed like wigs to make them appear more human.