Rinse the mahi-mahi and pat dry with paper towels. Cut the fish into 1-inch cubes.
In a shallow dish, combine the marinade ingredients and mix well.
Add the cubes of fish and toss to thoroughly coat. Cover and refrigerate for 1 hour.
If using bamboo skewers, soak them in a bowl of cold water for at least 30 minutes before grilling.
Meanwhile, halve the papaya and scoop out and discard the seeds, peel away the skin with a small knife or vegetable peeler.
Cut the papaya into 1-inch cubes. peel the plantain and cut across into 1-inch pieces.
Preheat the grill to high, or heat coals until they glow red, with white ash around the edges.
Take the fish cubes from the marinade, allowing excess to drip off; reserve the marinade.
Thread the fish onto the skewers, alternating with pieces of papaya and plantain. Do not crowd ingredients on the kebabs, but leave a little space between so they will cook evenly.
Rub a lightly oiled towel or cloth over the grill, add the kebabs to the hot grill and cook until well browned, 3 to 4 minutes. Brush a little marinade over the kebabs,
then turn and continue cooking until the fish is opaque through, about 5 minutes.