Merman And The half breed (Riren/Ereri)
Eren was a normal teenager, well that's what he though. What he never knew was he was half Merman. His mom was a mermaid and she fell in love with a human, who is known as Grisha Jaeger. Grisha loved his wife dearly until she left to go back to the ocean, leaving Eren with him. 10 years later, Grisha went out to sea to find her, but his ship crashed and everyone died, the last this left was a key. Eren turned the key into a necklace and he never leave it behind.
One day Eren was surfing with his friends Armin and Mikasa and he finds a strange cove. He swims in there, but he didn't realise the cove was filling with water. When it was filled to the top, Eren was trapped under and he couldn't breath, that was until a male with black hair and silver hair swims and finds him, but before Eren could get a good look, he passed out.