Floating Flower Tip - Most flowers float pretty well on their own until they start to pool with water and slowly sink. For a longer lasting centerpiece, use bubble wrap to make little skirts for your flowers.
How did I forget lilies? What are the tall, stock flowers among the callas? If you replaced them with two full sized peacock feathers and orchid stems instead of leaves on the bottom tier, this would be pretty much exactly what I Arrangement