Step 1: Combine 2 cups water, 1 tbsp fine sea salt, 2 tsp sugar in a shallow dish, stirring until sea salt and sugar dissolve; add 4 (6-ounce) rockfish fillets. Let stand 20 minutes. Drain; pat fish dry.
Step 2: Prepare charcoal fire in a chimney starter; let coals burn for 15 to 20 minutes or until flames die down. Carefully pour hot coals out of starter, and pile them onto one side of the grill. Coat grill grate with cooking spray; put grate in place over coals.
Step 3: Combine 1 1/2 cups finely chopped peaches (about 2 medium), 2 tbsp thinly sliced shallots, 1 1/2 tbsp fresh lemon juice, and 1 tbsp extra-virgin olive oil in a medium bowl; stir in 1/8 tsp table salt. Sprinkle 1/4 tsp table salt and pepper evenly over fish. Lightly coat fish with cooking spray. Place fish, skin side down, over direct heat on grill rack coated with cooking spray; grill 2 minutes. Turn fish over and move over indirect heat; grill 12 minutes or until desired degree of doneness. Stir mint into peach mixture. Serve with fish.