In a kettle, bring 6 cups water with the crab boil to a boil, simmer for 5 minutes, and add shrimp. Boil for 5 minutes and drain. In a small bowl, whisk together the vinegar, celery seeds, salt, and pepper to taste. Add oil in a stream, whisking continuously until emulsified. In a deep glass dish, arrange half the shrimp in one layer, arrange half the onion slices over them, and top with bay leaves. Arrange the rest of the shrimp over the onions and sprinkle the remaining onion slices over them. Pour the dressing over all. Let marinate, covered and chilled for 24 hours. Serve shrimp, skewered on picks or with crackers. A Gourmet Mag 1960 favorite.