Cut the fish in small bite-size pieces and put them in a bowl with lightly salted water.
Squeeze the limes. Then drain the fish and pat dry. Place the fish in a bowl and pour lime juice over it. Let it marinate at least 10 minutes (the lime "cooks" the fish, like a ceviche).
Meanwhile, prepare the vegetables: wash and dice or slice the tomato; peel, seed and dice the cucumber; peel, wash and finely grate the carrot; peel and mince the onion.
Drain the fish, discard the lime juice, and put the fish back into the bowl. Mix in the vegetables.
Pour the coconut milk, stir and season to taste.