Fresh curry leaves give a very South Indian taste to the curry but if you cannot find them, just leave them out.
Heat a small cast-iron frying pan over moderate heat. When hot, put the coriander seeds, fenugreek seeds and peppercorns and stir for about a minute or until they are lightly toasted. Remove from the heat. Place toasted seeds into a spice grinder or clean coffee grinder and grind as finely as possible.
Heat the oil in a large frying pan over medium heat. Add the mustard seeds and as soon as they begin to pop after a matter of seconds, add the curry leaves. Stir once and add the onion and garlic and stir until lightly browned. Add the ginger. Stir and cook for another few seconds. Now add the water, paprika, cayenne, turmeric, salt, chiles and the tamarind paste and bring to a boil. Lowering the heat to moderate and simmer vigorously for 5 minutes. Turn off the heat.
Five minutes before serving, heat the sauce over a fairly high flame. As soon as it begins to bubble, add the shrimp and stir until they begin to turn opaque. Stir the coconut milk add it to the pan, stirring constantly. When the coconut milk is heated through and the first bubbles begin to appear, turn off the heat and serve.