In a saucepan, boil 8 cups of water with salt. Add shrimp and boil for about 2 minutes until cooked. Drain and chill in an ice bath. Drain again.
In a medium bowl, whisk olive oil with lemon juice. In 1 large or 2 smaller glass bowls, layer the shrimp, garlic, onion, bay leaves parsley and 6 of the lemon halves. Add the crab boil seasoning and the lemon dressing, stir to combine and cover. Marinate in the refrigerator for 12-24 hours.