Preheat a grill pan over medium heat.
Using a sharp knife, make two vertical slices through the skin of fish fillets, taking care not to cut the flesh; season with sea salt and pepper.
Place fillets on grill, skin-side down, and grill until skin is crisp and fish is almost cooked through, about 3 minutes. Carefully turn fish and continue grilling until cooked through, 1 1/2 to 2 minutes. Sprinkle fish with Maldon sea salt and serve immediately with lemon wedges.