boat is on a diagonal object slicing through the composition
disciples are flailing about while Jesus naps serenely
the water is churning, dark, angry, threatening as it dips and rocks the boat throwing the disciples off balance but not Jesus
Jesus is the only one illuminated by a halo though it was customary to put halos on the disciples also but by this time in art, nobody usually had a halo in religious painting; in Delacroix's time, halos were old fashioned and he used it deliberately
The blue of Jesus's cloak is a traditional symbol of purity. The cloak has a deep shadow of blue similar to the raging water and dark sky above which is an ominous suggestion of what's to come in his life.
