Prepare shrimp topping first: Combine 2 Tbsp oil and flour with a wire whisk and cook in a heavy skillet until light brown. Set mixture aside.
Cook celery, green pepper, and onion in 1 Tbsp oil in skillet over med-high heat stirring constantly for 1 minute. Add shrimp and garlic; cook 1 minute, stirring constantly.
Stir in water and remaining ingredients. Heat to bubbling. Gradually stir in reserved flour mixture, stirring until smooth.
Cook until thoroughly heated, then remove from heat but keep warm.
Catfish: In a medium bowl combine eggs and next 3 ingredients, plus 2 tsp cajun seasoning and set aside.
In a shallow dish combine flour and remaining 2 tsp cajun seasoning.
Dredge each fillet in flour, then in the egg mixture and fry in hot oil 5 minutes each side, or until golden. (fry in batches, if necessary).
Remove from oil and drain on paper towel.
Serve over hot rice with shrimp topping spooned onto each fillet.
Garnish with chopped fresh parsley or scallions, and a lemon wedge.