Divide crabmeat into sizable flakes.
Cut celery in very thin slices.
Toss crabmeat and celery together.
Chill in refrigerator until sauce is made.
Crush garlic and mix with remaining ingredients.
Shortly before serving, mix crabmeat with about two thirds of sauce.
Transfer to salad platter or salad bowl, and garnish with watercress, thin slices of peeled tomatoes, and thin slices of cucumber.
Serve remaining sauce in bowl.