To make the watermelon salsa, combine the watermelon, corn, red onion, cilantro, lime juice and jalapeno in a bowl. Toss with 1 tablespoon olive oil and ½ teaspoon salt and set aside.

Preheat a grill to high. Sprinkle the fish on both sides with the chilli powder and ½ teaspoon salt. drizzle both sides with the remaining 2 teaspoons olive oil. Brush the grill with olive oil, then add the fish and grill until cooked through, 4 to 5 minutes per side. Transfer the fish to a plate and break into bite-size pieces.

Meanwhile, toss the lettuce with 2 tablespoons of the juices from the watermelon salsa and a pinch of salt. Warm the tortillas on the grill and fill with the fish, watermelon salsa, and lettuce.

Cut the avocado into cubes, the cherry tomatoes in half, the jalapeno pepper in slices and sprinkled them along with some cilantro leaves over the fish tacos. Serve with lime wedges.