Combine first 3 ingredients in a large saucepan. Bring to a boil, and cook until reduced to 1/2 cup (about 20 minutes). Cool. Combine one-fourth pineapple juice mixture, yogurt, vinegar, salt, and pepper in a small bowl. Chill dressing.
Place pineapple on grill rack coated with cooking spray, and grill 2 minutes on each side.
Place crab on grill rack coated with cooking spray. Grill 5 minutes; turn and baste frequently with yogurt mixture.
Divide spinach and watercress evenly among 6 plates. Top each serving with 2 pineapple pieces, 1/3 cup onion, 1/3 cup avocado, and 1 crab; drizzle each with 2 tablespoons dressing.