Not every fairy tale ends the way you think.
Prince Finlay is out on the high seas, on his way to the court of his cousin, when he sees what he thinks is a woman drowning. When he calls for help and the captain arrives, there's nothing in the water. Finlay knows what he saw, but he can't explain where the woman went.
As the crew exchanges stories at dinner, Finlay listens intently to one told of a mermaid who took a sailor captive as her pleasure toy. When the sailor mentions the red hair of the mermaid, it reminds Finlay of the woman he saw in the water. Leaving the cabin to get some fresh air, Finlay sees the red-haired woman in the water again.
She convinces him to join her in the water, but he quickly realizes all is not what it seems.