Combine water, 2 teaspoons lemon juice, and 1/4 teaspoon salt in a saucepan. Bring to a boil over high heat. Stir in couscous. Remove from heat; cover and let stand 5 minutes. Place couscous in a bowl; stir in parsley and next 3 ingredients.
Brush tuna with 2 teaspoons oil and 1/4 teaspoon salt. Place fish on grill rack coated with cooking spray. Cover and grill 2 minutes on each side or until desired degree of doneness. Remove from grill, and flake into large pieces.
Whisk together 2 tablespoons olive oil, remaining lemon juice, and 3/4 teaspoon salt. Pour vinaigrette over couscous mixture. Add tuna, and toss. Sprinkle with pepper.