Rinse shrimp and scallops. Fill a bowl with ice water.
In a 3- to 4-quart pan over high heat, bring about 2 quarts water to a boil. Add the shrimp, reduce heat to maintain a simmer, and cook until shrimp are opaque but still moist-looking in center of thickest part (cut to test), 1 to 2 minutes. Remove with a strainer or a slotted spoon and immerse in ice water until cool. Add scallops to simmering water and cook until opaque but still moist-looking in the center (cut to test), 1 to 2 minutes; lift out and immerse in ice water until cool. Drain shrimp and scallops. Cut each shrimp crosswise into thirds.
Meanwhile, in a glass or ceramic bowl, mix coconut milk, lime juice, bell pepper, chilies, cilantro, ginger, and salt. Stir in shrimp and scallops. Cover and chill at least 6 hours or up to 1 day.
Spoon into a large serving bowl or onto a rimmed platter.