Preheat the broiler to high. Line a broiler pan with foil and lightly oil.
Skim 2 tablespoons cream from the top of the coconut milk (reserve the milk to flavor rice as a side dish, if desired). Fry the coconut cream in a small skillet over medium heat until glossy, about 2 minutes. Stir in the curry paste and cook until fragrant, 2 minutes. Spread the mixture over the fish in the broiler pan; broil until browned and just cooked through, about 15 minutes. Cut the fish into 4 pieces.
Whisk the lime juice, sugar, 2 tablespoons water and fish sauce in a bowl until the sugar dissolves. Stir in the chili sauce and garlic.
Arrange the lettuce, watercress, cucumber, scallions, pepper, herbs, nuts and lime wedges with 4 pieces of fish on a platter; drizzle with the Thai salad dressing.