Moana helping the turtle as a child was a metaphor for how the ocean helped her throughout the film.

Moana didn’t pick up the turtle and carry it to the ocean, which would have been easiest for the both of them. She protected the turtle and let it make the journey itself. Because it was important that the turtle got to the ocean on its own.

This is why the ocean chose Moana. Even at a young age, she understood why the turtle needed to make its own journey.

Throughout the film, the ocean guides and protects her along the way, but it most certainly does not carry her to return the heart of the sea. It is important for Moana to make the journey by herself. She needs to fight her own battles, endure her own hardships, and forge her own path so she may become stronger.