You were a mermaid and whenever you touched water you transformed. One night Will dived into the moon pool, on Mako island, also your spot. Because of the full moon he hit his head hard and you saved him.

The both of you were on the shore, he started talking about Mako only. After some time he understood that you were hiding something so he spilled water on you and you had no choice but to jump in the water and he came after you seeing your tail. After you got out

"Why didn't you tell me?"