For the light and elegant pesce all’acqua pazza, simplicity—and skin—beget success. “Acqua pazza,” or “crazy water,” refers to the southern Italian tradition of cooking the day's catch in seawater. Modern recipes feature longer ingredient lists: Parsley, tomato, white wine, and garlic are common. For the fish, we chose skin-on haddock fillets. The flesh held up nicely during simmering, and the abundant collagen in the skin suffused the broth with rich flavor and body. Halved cherry tomatoes…