Wash the sea bass thoroughly and set aside. Separate and wash the celery stalks, reserving the leafy parts. Remove the strings and cut the stalks into pieces about 4 cm (1½ inches) in length. Bring a large pan of salted water to the boil and blanch the celery for 2 minutes. Meanwhile, finely chop the reserved leafy parts.
Put the passata into a frying pan with the paprika, pepper, a little salt and half the olive oil. Heat gently for 10 minutes, then add the blanched celery and leafy parts and continue to cook for a further 10 minutes. Remove from the heat and allow to cool.
Preheat the oven to 180°C (350°F), Gas Mark 6.
Put half the tomato sauce in the base of an ovenproof dish that will accommodate the fish and place the sea bass on top. Drizzle over the remaining olive oil and the rest of the sauce, then add the water. Mix well, then cook in the preheated oven for around 45 minutes. Serve with couscous.