Debussy was inspired to write this piece by an ancient legend of the Cathedral of Ys, told by the people of the Brittany section of France. When analyzing this piece it can be helpful to look at the famus painting of the Cathedral itself. In it you can see the very outline of the song written by Debussy.
I found that the over all for of the piece is ABCBA. In the painting there is almost a mirror efect. In the middle of the paining is a horizontal line which devides the sky, and the ocean. on the line is the Cathedral. It is a pictur of the songs for. The form is known as an arch form. reflection. The piece is devided at measure 28, 47, 72, and 84. there are transitions from theme to theme. The first is from m26-28. The C section starts at m47 with the key change. Then, just like a reflection, it returns back to its origonal key in measure 72.
Also... He paints a pictur in the begining of church bells ringing which make the listener think of the church, on the quiet shore with bells ringing.