You're the mermaid princess. You got lost. Your stranded on a rock because you got stuck in your human form. the straw hats are sailing and Usopp sees you and that you are unconcious on the rock. Usopp alerts the others. But they don't recognize you since you look like a normal human. So they sail over to you and bring you aboard the ship. Everyone except Sanji is is sleeping, Sanji is watching you to make sure you are safe (And also because he is a huge simp over beautiful girls).

Sanji: I wonder when she will wake up...