Marinate the Fish: In a medium bowl, combine 1/2 cup coconut milk, the zest and juice of 1 lime, salt, pepper, and if desired, a pinch of cayenne pepper. Mix well. Place 4 white fish fillets in a shallow dish and pour the marinade over the fish, making sure they are well-coated. Cover and refrigerate for 15-20 minutes.
Prepare the Salsa: While the fish is marinating, take a mixing bowl and add 1 diced avocado, 1/2 diced red onion, 1/2 cup halved cherry tomatoes, 1/4 cup chopped cilantro, and the juice from 1/2 lime. Gently toss these ingredients together and season with salt and pepper to taste. Set aside to let the flavors mingle.
Cook the Fish: Heat 1 tablespoon oil in a skillet over medium heat. Once the oil is hot, gently place the marinated fish fillets in the skillet. Cook for about 3-4 minutes per side or until the fish is cooked through and flakes easily with a fork. If using a digital thermometer, the fish should reach an internal temperature of 145°F (63°C).
Serve: Once the fish is cooked, remove it from the skillet and place it on serving plates. Top each fillet generously with the prepared avocado salsa and garnish with additional cilantro and lime wedges to serve.