Merpeople are very dangerous and untrusting of those on land especially because of so much pollution of the seas and overfishing. They can breathe both in and out of the water, but their scales dry when they are out. They do get very curious and can come close to shore. They are usually seen at night on docks or on the beach. Sometimes they have been seen during the day on rocks or trapped in tidal pools. They live in loose societies.  They vary greatly according to where they are located and they will share characteristics with the fish that they live near.  Mermen are solitary and rare. They are larger in size than the females. You can tell if a merfolk has been near by a pile of shellfish shells laying on the beach as well as a sharp instrument used for prying open those shells. Also merfolk use sheets of seaweed for clothing, rope and baskets that sometimes can be found drying on the shore. 
* photos courtesy of Arthur Spiderwick's Field Guide