Combine curry powder, ground ginger, and ground turmeric in a skillet; toast the spices over medium heat until browned, about 5 minutes. Pour olive oil and garlic into the spice mixture and stir to make a paste.
Stir onion into the spice mixture; cook and stir until the onion is tender, 5 to 7 minutes. Stir about half the coconut milk and half the water into the onion mixture; simmer together for 5 minutes. Add cod to the mixture; continue cooking until the fish is firm, about 5 minutes. Add tomato, remaining coconut milk, and remaining water to the skillet; cook until the tomato is softened, about 5 minutes.
