In a medium skillet, combine 1/3 cup water and oil. Add the fennel, tomatoes, garlic, 1/2 teaspoon salt, lemon, and 1/4 teaspoon pepper. Cover, and cook, stirring occasionally, until tomatoes begin to fall apart, 5 to 7 minutes.
Season the fish with salt and pepper. Add to skillet with vegetables. Cover, and cook until opaque, 10 to 15 minutes. Serve immediately.