Use any firm fish here, and whatever shrimp or crab you have available. I like making a fish stock from the heads and bones of the fish I am using, but you can skip that and use chicken stock instead, or even water. Use coconut water for the water in the stew if you can get it, otherwise just skip it. Serve this fish stew with rice.
In a large, heavy pot like a Dutch oven, heat the oil over medium-high heat and saute the onions until soft, but not browned.
Add the tomato sauce, curry powder and potatoes and stir well. Pour in about 6 cups of the broth you just made, or chicken stock. Bring to a simmer and taste for salt. Simmer gently until the potatoes are almost tender, about 20 minutes.
Add the sweet peppers, the chiles, the diced tomato and the coconut milk. Stir well and simmer gently for 10 minutes.
Add the fish and shrimp and simmer until the shrimp are nice and pink, about 5 to 10 minutes. Add the cilantro, cook another minute, then serve.