Peel shrimp, reserving shells, and set shrimp aside.
Melt 1 teaspoon butter in a large Dutch oven over medium heat. Add reserved shrimp shells, onion, celery, carrot, thyme, and garlic; sauté 3 minutes. Add tomato, water, wine, and tomato paste; bring to a boil. Reduce heat, and simmer, uncovered, 35 minutes. Place half of seafood broth in a blender, and process until smooth. Strain pureed seafood broth through a sieve into a bowl; discard solids. Repeat process with remaining seafood broth.
Combine 1 cup seafood broth, 1 teaspoon butter, salt, and pepper in a medium saucepan, and bring to a boil; gradually stir in couscous. Remove from heat; cover and let stand 5 minutes. Fluff with a fork before serving.
Heat oil in a large nonstick skillet over medium-high heat. Add shrimp, and sauté 1 minute. Add bell peppers, squash, and zucchini; sauté 3 minutes. Stir in couscous and an additional 1 1/2 cups seafood broth; cook 1 minute.
Note: Store remaining seafood broth in an airtight container in refrigerator for up to 1 week, or freeze for up to three months.