Peel shrimp; place shells in a saucepan. (Refrigerate shrimp until ready to use.) Add celery ends and onion peel to pan; chop remaining celery and onion, and reserve. (Using the leftover bits of onion and celery will layer the flavor and result in a flavorful broth.) Add broth; bring to a boil over medium-high heat. Reduce heat to low; simmer 30 minutes.
Meanwhile, cook sausage in a large Dutch oven over medium-high heat, stirring often, 7 to 8 minutes or until browned. Remove sausage; pat dry. Wipe Dutch oven clean. Sauté celery, onion, and peppers in hot oil in Dutch oven over medium-high heat 5 to 7 minutes or until onion is tender. Add garlic, and sauté 45 seconds to 1 minute or until fragrant. Stir in potatoes, next 4 ingredients, and sausage.
Pour broth mixture through a fine wire-mesh strainer into Dutch oven, discarding solids. Increase heat to high, and bring to a boil. Reduce heat to low, and cook, stirring occasionally, 20 minutes or until potatoes are tender.
Add fish; cook 2 to 3 minutes or until just opaque. Add shrimp, and cook 2 to 3 minutes or just until shrimp turn pink. If desired, stir in crawfish, and cook 2 to 3 minutes or until hot. Add salt and pepper to taste.
Spoon seafood into warmed soup bowls. Top with broth mixture. Serve immediately with Griddle Corn Cakes and Spiced Mayonnaise.