Pesach - Lessons of the Song of the Sea

Why does the song of the sea focus on the 'raising of the honor of the people of Israel' and on the destruction of Pharaoh's legions in the sea? Why does Yisro come with a completely different view of the Exodus - the degree of Hashem's justice, which is measure for measure? Why is that lesson only taught later to the Jewish people at Mara? What is the lesson in Hashem creating miracles where bitter things are sweetened only by other bitter things?