Place all the dried spices ingredients in a small frying pan over medium-high heat. Dry roast, stirring or shaking the pan regularly for 1 to 2 minutes so they won't burn. Allow the spices to cool for 5 minutes.
Transfer the spices into a spice grinder or mortar and pestle and work/process until it's fine powder consistency. A small powerful food processor would work too, but the spices will be coarser.
Heat oil in a large deep frying pan over medium-high heat. Place in the garlic, onion, ginger, curry leaves and chillies, stirring quickly for 1 to 2 minutes or until fragrant. Add in the dried spices, tomatoes, and turmeric, then stir for 2 minutes.
Pour in the coconut milk and water, then bring to a boil. Stir in the tamarind, sugar and salt then decrease the heat to low. Simmer for 10 to 15 minutes or until the sauce thickens.
Increase the heat to medium-high, then add in the fish and prawns. Stir gently just until the seafood turns opaque (3 to 4 minutes). If the curry is too thick, add a few tablespoons of water. Have a taste of the curry and add more tamarind, salt, or sugar to your liking if needed. Remove the pan from the heat and stir in the coriander. Serve with rice, roti or naan bread.