Fill a large stockpot with about 15 quarts water. Add salt, lemons, crawfish boil, garlic, cayenne, and bay leaves; bring to a boil over high heat. Reduce to a simmer; cook for 15 minutes.
Add potatoes, corn, and onions. Return to a boil; cook for 5 minutes.
Add crawfish, and return to a boil. Cook for 10 minutes.
Using a large slotted spoon, transfer crawfish and vegetables to a large serving platter. Season with salt and pepper. To eat, hold crawfish in one hand; gently twist off tail. Peel off first 2 or 3 rings; suck juices out of the head. Squeeze sides at top of tail with thumb and forefinger to break shell; remove meat.