In a large bowl, combine the fish cubes with lime juice. Marinate for about 15-20 minutes. The acidity from the lime juice will "cook" the fish slightly.
While the fish is marinating, prepare the vegetables. In a separate bowl, combine the chopped red onion, diced cucumber, diced tomato, and sliced chili peppers (if using).
Drain the excess lime juice from the fish and discard it.
Add the coconut milk to the marinated fish and mix well.
Stir in the prepared vegetables and chopped cilantro. Season with salt and black pepper to taste.
Enjoy your meal!