Preheat a two-burner grill pan over medium-high heat. In a small bowl, combine the piña colada mix and soy sauce. Thread the pineapple, mahi mahi, bell peppers and onion onto eight 12-inch skewers; place a cherry at the tip of each. Brush the kebabs with oil and season with salt and pepper.
Place the kebabs on the grill pan, pressing to mark, and cook, turning occasionally and brushing with two-thirds of the piña colada-soy mixture during the last 2 minutes, until the fish is well marked and opaque in the center, about 8 minutes. Transfer to a platter and baste with the remaining piña colada-soy mixture.