For Cajun spice mix, combine thyme, white pepper, salt, black pepper and red pepper in a small bowl. Set aside.
Thaw shrimp, if frozen. Peel and devein the shrimp, leaving tails intact if desired. Rinse the shrimp; pat dry with paper towels. Cook flour in a medium skillet over medium heat about 6 minutes or until the flour is browned, stirring frequently. Place in a small bowl; set aside to cool.
Heat oil in a large pot over medium-high heat. Add onion, bell pepper, celery, and garlic; cook and stir about 5 minutes or until vegetables are tender.
Slowly whisk 1 can of the broth into the reserved browned flour. Add the broth-flour mixture, the remaining 1 can broth, the water, and the reserved spice mix to the mixture in the large pot. Stir in okra. Bring to boiling; reduce heat. Cover and simmer for 15 minutes.
Add the shrimp; cook for 2 to 3 minutes or until the shrimp is opaque. Gently stir in crabmeat. Serve the gumbo with rice. If desired, garnish individual servings with scallions and pass hot pepper sauce.