Remove the white pith from the grapefruit. Working over a bowl to catch any juice, separate the grapefruit segments. Reserve any collected juice for the vinaigrette.
Halve the endives lengthwise, and core; separating leaves. Cut avocado into 1/4-inch-thick slices. Arrange the grapefruit segments, endive, sliced avocado, red onion, and shrimp in a large serving bowl. Season with salt and pepper and gently toss.
For the Vinaigrette Dressing:
In a medium bowl, whisk reserved grapefruit juice, vinegar, garlic, salt and pepper. Slowly beat in the oil, whisking constantly.
Drizzle dressing over salad and garnish with watercress, serve immediately.