She didn't mean it. She was angry at the man and shocked. Now everything is blood and screaming. The man will be eaten. She doesn't want any of him. She only wanted to touch his things and see him. She was having fun. Then he was angry at her for nothing. And now this.
The red haired one bites him deeply in the shoulder and then uses him to pull herself up into the boat on top of him. The boat is rocking fit to tip over, swinging all four of them back and forth. The red head has never been close to a sword, only been shot at. She doesn't know when she leaps onto Jack. It looks so flimsy. But it goes straight through her side, impaling her there. Only sea water splashes over Jack's hand. Her face snaps at his from inches away. The bites come slower as her eyes fade and she dies without knowing she was even wounded badly.
The ruined black haired mermaid throws herself back into the water and vanishes. Still inside the boat, the blonde shakes facedown in the boat, covering her eyes and waiting for the blow.