Preheat a grill. Toss the squid bodies with the oil, and generously season with salt and lots of black pepper. When the grill is very hot, add the squid and grill for about 5 minutes, until lightly charred all over. Combine the lime juice, fish sauce, sugar, and chili sauce in a mixing bowl and whisk to blend. Slice the grilled squid into 1⁄2" rings. In a salad bowl, toss the squid, watercress, cucumber, tomato, onion, and peanuts with the dressing. Divide the salad among 4 plates.