Crab and Shrimp Salad with Avocado and Oranges

Name: Crab and Shrimp Salad with Avocado and Oranges
Ingredients: 1 small red onion, cut crosswise into slices 1/8 inch (3 mm) thick; 1/4 cup (2 fl. oz./60 ml) red wine vinegar; Kosher salt, to taste; Sugar, to taste; 12 to 18 large shrimp in the shell; 2 large navel oranges; 1 Tbs. sherry vinegar; 1/2 Tbs. fresh lemon juice, plus more as needed; Freshly ground white pepper, to taste; 6 Tbs. (3 fl. oz./90 ml) extra-virgin olive oil; 2 bunches watercress, tough stems removed; 1 head red-leaf lettuce, leaves separated and torn into large pieces; 8 to 12 oz. (250 to 375 g) cooked crabmeat, picked over for shell fragments; 1 large ripe avocado, pitted, peeled and diced