Preheat a gas grill to high heat with the lid down. Brush the crabs with the olive oil and season with salt and pepper. Place the crabs on the grill and cover, turning every few minutes and basting with the olive oil until they begin to turn orange and the crabs are cooked through, about 7 to 9 minutes, depending on their size. Remove from the grill, and serve with the salad.
In a medium bowl, combine the grapefruit juice, honey, shallot, mustard, and garlic. Use a whisk to blend. While whisking the ingredients in the bowl, slowly drizzle the oil a few drops at a time into the bowl. Once all of the oil has been incorporated into the bowl, stir the basil into the vinaigrette and season with salt and pepper to taste.
Place the watercress in a large bowl, and add the mango, avocado, basil chiffonade, and 1/4 cup of the vinaigrette; toss gently and season with salt and pepper. Divide among 6 salad plates. Place crab on top. Drizzle 2 teaspoons of the remaining vinaigrette over each of the crabs, and serve immediately.