1. Combine 2 cups water, sea salt, and sugar in a shallow dish; add fish. Let stand 20 minutes. Drain.
2. Prepare charcoal fire in a chimney starter; let coals burn until flames die down. Pour hot coals out of starter; pile on one side of grill. Coat grill grate with cooking spray; put grate in place over coals.
3. Sprinkle 1/4 teaspoon salt and pepper over fish flesh. Divide dill and lime slices evenly among fish cavities. Coat outside of fish with cooking spray. Place fish over direct heat; grill 4 minutes. Turn over; move to indirect heat. Grill 12 minutes or until done.