In a large bowl, combine the shrimp, cod fillet, garlic, lime juice, salt, pepper and chili pepper flakes; set aside, preferably in the refrigerator.
Next, in a large 4½ quart Dutch oven or medium saucepan, heat the olive oil over medium heat. Add the onion and cook, stirring occasionally, until just translucent and softened, about 2 minutes. Add the bell peppers and continue cooking for about 2 minutes, stirring occasionally, until softened. Add the fish stock and cook for an additional minute.
Add the shrimp and chunks of cod, along with their marinade, followed by the diced tomatoes and cook for about 8 to 10 minutes, stirring very delicately from time to time, until the shrimp and fish are just opaque throughout and the mixture has released plenty of liquid.
Dilute the tapioca starch in the water and delicately stir that in the chowder; bring back to a simmer, cook for a full minute and then kill the heat.
Delicately stir in the coconut milk and chopped cilantro and serve without delay.