Cut the fish into 1 in pieces and peel the shrimp. Set aside.
Heat the oil in a large sauté pan. Fry the onion over medium-high heat, until golden brown, 4-5 min. Stir in the garlic and ginger. Sauté for 1 min.
Add the diced tomatoes, diced potatoes, seasonings (except salt), and water. Stir to mix in the spices. Simmer the mixture, covered, over medium heat for 15-20 minutes, until the potatoes are very tender.
Add the salt and mix well. Add the fish and prawns and stir. Simmer, uncovered for 10 minutes, until the seafood is opaque and cooked through.