I looked up the dictionary, it described evanescent as vanishing, or passing away like vapor; and I knew departure is separation. Somehow I couldn't figure this sentence, and its relationship with the ocean.
And, ah! his castle. The faery solitude of the place; with its turrets of misty blue, its courtyard, its spiked gate, his castle that lay on the very bosom of the sea with seabirds mewing about its attics, the casements opening on to the green and purple, evanescent departures of the ocean, cut off by the tide from land for half a day. . .