To make the soup base: In large saucepan over medium heat, combine the water, canned tomatoes, wine, leeks, carrots, onion, thyme, lemon, saffron, and bring to a boil. Lower the heat to a lively simmer and cook until reduced by about one-third, about 45 minutes. Stir in olive oil, season the mixture lightly with salt and continue to simmer until the liquid portion of the soup base is reduced to about 8 cups, about 20 minutes. Strain the soup base into a 3-quart saucepan and keep it warm over low heat. Discard the solids. The soup base may be prepared up to 3 days in advance and refrigerated.
To prepare the soup: If you have prepared the soup base in advance, bring it to a simmer in a medium saucepan. Adjust the heat to very low and keep warm. In a large (about 8-quart), heavy saucepot over medium heat, add the olive oil and heat, about 1 minute. Add the sliced onions, trimmed leeks and garlic cloves. Cook the mixture, stirring, until the onion is wilted but still crunchy, about 4 minutes. Add the calamari rings and cook, stirring, until they turn opaque, about 2 minutes. Pour in all but 1 cup of the hot soup base and bring to a boil. Stir in the scallops, fish fillets and beans, if using. Adjust the heat to simmering and cook until the seafood is barely opaque at the center, about 5 minutes.
Meanwhile, add the mussels to the soup base remaining in the saucepan. Increase the heat to high, cover the saucepan, and steam over medium heat, shaking the pan occasionally, until the mussels open, about 3 minutes. Discard any mussels that do not open on their own.
Stir the shrimp, parsley and steamed mussels into the large pot of soup. Simmer until the shrimp is cooked through, about 1 minute. Check the seasoning, adding salt, if necessary, and freshly ground black pepper. Garnish with chopped parsley. Ladle soup into warmed soup bowls. Serve with crusty Italian bread.
Braised Cannellini Beans: In a large bowl, add the beans and enough cold water to cover them by 4 inches. Let soak in a cool place or in the refrigerator for at least 8 hours or overnight.
Drain the beans and transfer them to a 2-quart saucepan. Pour in enough water to cover by two fingers and drop in two of the rosemary sprigs. Bring the water to a boil, and then lower the heat so the water is at a bare simmer. Cook until the beans are tender but not mushy, with just enough liquid to cover them, 30 to 40 minutes. (If necessary, add more water a tablespoon at a time to keep the beans covered as they simmer.)
Remove the beans from the heat and gently stir in the oil, salt to taste, and the remaining 2 rosemary sprigs. Let the beans stand to cool and absorb the cooking liquid. The end result should be tender beans with a creamy consistency in just enough liquid to coat them. As the beans cool, taste and season with salt, if necessary.