Why does Lady Capulet say this to Juliet?

She is saying that an unmarried man is like an unbound book about love. It may be beautiful on the inside once it is read but people can't see that beauty by looking at the plain pages. She is telling her daughter that if she marries him, she will be the beautiful cover to the book, and they will both be more highly thought of by people by being together. The cover gets its value from the book inside, and the book is more highly thought of with a beautiful cover. The reference to fish is the same idea: a fair creature lives in a beautiful sea.