Bring 3 cups of the water to a boil. Add salt to taste and the barley, and simmer, uncovered until just tender, 35 to 40 minutes. Drain, and set aside.
Combine the clam juice, wine, remaining ½ cup water, lemon slices, thyme, and parsley in a medium-size heavy saucepan. Heat to boiling, then reduce the heat. Add the lobster tail and gently poach until pink and firm, 3 to 5 minutes. Using a slotted spoon, transfer it to a plate. Add the shrimp to the saucepan; cook 3 minutes. Transfer them to the plate. Add the scallops to the saucepan and poach until barely firm, about 3 minutes. Transfer them to a bowl. Raise the heat under the saucepan and reduce the stock to ¼ cup. Strain and reserve.
Clean and devein the shrimp and cut each in half crossways. Clean the lobster tail and cut into pieces.
In a large bowl, combine the barley, scallions, cucumber, zucchini, radishes, red bell pepper, basil and the seafood. Toss well.
In another bowl, use the back of a spoon to mash the garlic and ½ teaspoon salt together to form a paste. Stir in the mustard and the reserved fish stock. Whisk in the oil and lemon juice. Pour this dressing over the barley-seafood mixture, and mix well. Serves 6 to 8.