In a 7- to 8-gallon kettle bring 5 gallons water to a boil.
While water is coming to a boil, in a deep sink rinse crabs in 2 or 3 changes of water. (Do not fill sink too full or crabs may crawl out.)
Add Zatarain's crab boil or 5 tablespoons Old Bay seasoning, cayenne, salt, lemons, and onions to boiling water and boil 5 minutes. Add potatoes and garlic and boil until tender, about 15 minutes. Transfer potatoes and garlic with a large sieve to a large platter and keep warm, covered with foil. Add corn to boiling water and cook until tender, about 5 minutes. Transfer corn with tongs to platter and keep warm, covered with foil.
Return water to a boil. Transfer about 6 crabs carefully with long tongs to sieve, holding them down in sieve with tongs, and turn crabs out into boiling water. Add remaining crabs in same manner. Return water to a boil and cook crabs until just cooked through, about 10 minutes. (To determine doneness, remove a claw and crack it with the handle of a dinner knife. Use Knife to extract meat from claw.) Transfer crabs with sieve to another large platter and sprinkle with 1 tablespoon Old Bay seasoning.
Return water to boil and cook shrimp until just cooked through, about 2 minutes. Transfer shrimp with sieve to large platter with crabs and sprinkle with remaining tablespoon Old Bay seasoning.
Serve crabs, shrimp, and vegetables with horseradish cocktail sauce and French bread.