Grate 1/4 teaspoon zest from the grapefruit and set aside. Trim the ends from the grapefruit, then cut away the peel and white pith. Working over a bowl, cut along both sides of each membrane with a paring knife to remove the segments. Discard any seeds. Cut the grapefruit segments into pieces. Squeeze the juice from the membranes into the bowl.
Make the dressing: Whisk the sour cream, 2 tablespoons of the grapefruit juice, the grapefruit zest, scallions, 2 tablespoons water and 1/2 teaspoon each salt and pepper in a bowl.
Toss the watercress, endive, lettuce, celery and onion in a large bowl. Add the dressing and toss to coat. Divide the salad among shallow bowls. Top with the reserved grapefruit segments and the smoked trout.