Mary isn't like the other mermaids in her pod. She grew up with certain disabilities. She can't hold her breath for as long as the others, so she has a rebreather to help her. Her tail was malformed, coming down in two parts instead of one, so she can't swim as fast as the others. But the thing that made Mary the most different was that she was adopted... after being found in a human vessel.
But when the humans come back looking for Mary, she is forced to make the biggest decision, go with them or risk her family and the secret that had kept her people safe for centuries. Safe from the monsters that lived on shore. Safe from her and the people that she was born to. It wasn't that hard of a decision. What was hard was figuring out how to survive in that world and how to get back home.