Season the sea bass with salt and pepper; arrange in a baking dish and set aside.
Melt the butter with the olive oil in a skillet over medium-low heat. Cook the shallots, garlic, and parsley in the melted butter and olive oil until softened, about 5 minutes. Add the flour and stir continually until completely incorporated into the mixture and the liquid is smooth. Pour the half-and-half into the mixture while stirring; cook and stir until thickened. Season with the white pepper. Gently fold the crawfish tails and crabmeat into the mixture, taking care to not break the crabmeat too much; cook another 1 minute. Spoon the mixture over the sea bass fillets.
Bake in the preheated oven until the fish flakes easily with a fork, 15 to 20 minutes. Serve immediately.