Begin by brining your shrimp. Combine water, sugar, and salt in a large bowl and stir. Leaving the head and tails of the shrimp intact, place them in the large bowl full of brining liquid and let them sit for 30 minutes untouched at room temperature.
While the shrimp is brining, make your aioli sauce. Place the eggs, pepper, salt, lemon juice, clam juice (optional), and garlic in a cuisinart and blend. With motor running, slowly add in the oil in a thin stream. Process until it reaches your desired consistency, which shouldn't be long after you finish adding in the oil. Remove the aioli and store in the refrigerator until you are ready to use.
After the shrimp are done brining, drain them. Bring a large pot of water, a lemon, and a good amount of pepper to a boil. Place the shrimp in the pot and cook for about 2 minutes, then remove and place into an ice bath. Do not overcook the shrimp - 2-3 minutes for medium size shrimp is all you need! Peel and devein the shrimp, and chop into small pieces.
Combine the shrimp, crab meat, and celery in a large bowl and season lightly with salt and pepper. Add in a few tbsp. of the aioli, and stir well.
Slice your sourdough bread into 1/2 inch pieces, and butter each side. Grill the bread or toast it in the oven. Top the bread with a touch more aioli and the shrimp and crab mixture. Garnish with lemon zest, capers, parsley, and whatever else you like. Enjoy!