What does the spirit mean by what belongs to the sea will always return to the sea if it is a mystery and you are not supposed to know yet please tell?

Answer 1

If you are directly at the part where she says that, the answer will be revealed later in the story, and do not read my answer below, for it contains spoilers for later in the book. But if not, and you have finished the book, read on!

Well, I believe that it means exactly that literally. Because when they crushed the pearls they were transported out of the underworld and into the sea. And since the pearls were given to them by a spirit of the sea, what belongs to the sea, returns to the sea.