Gather the ingredients.

Heat olive oil in a large saucepan; add onions, leeks, chopped tomatoes, and garlic. Cook and stir over low heat until vegetables have softened, about 3 to 5 minutes.

Stir in fennel, thyme, bay leaf, and orange zest. Add mussels, shrimp, and boiling water; stir to combine. Season to taste with salt and black pepper. Increase the heat to high, and boil for about 3 minutes to allow the oil and water to combine.

Add seabass and reduce the heat to medium. Continue cooking until fish is opaque and tender, but still firm, about 12 to 15 minutes. Fish should not be falling apart.

Taste the bouillabaisse and adjust the seasoning. Stir in saffron, and then pour soup into a warmed tureen or soup dishes. Serve immediately.