Peel shrimp; devein, if desired. Drain and flake crabmeat, removing any bits of shell. Set seafood aside.
Sauté onion, garlic, and celery in hot oil in a Dutch oven over medium-high heat 8 minutes or until tender. Stir in flour, and cook, stirring constantly, 1 minute. Stir in broth and next 4 ingredients. Bring to a boil; cover, reduce heat, and simmer, stirring occasionally, 30 minutes or until potatoes are tender.
Stir in shrimp, crabmeat, and heavy cream; cook over low heat 5 minutes or just until shrimp turn pink. Garnish, if desired.