He fell. One of his crew members thought it would be funny to mess around in crashing waters, and he fell which made him get washed away from his ship.

As he was in the water, he swore he felt the delicate arms of someone carry him to keep him up the ocean. He would swear it on his life he saw shining scales and got a glimpse of your face. You weren't a mermaid though..you looked far more different.

But the overbearing salt water got in his lungs, and he passed out before he knew.