In a very large stockpot (at least 12-quart), combine the water, Old Bay seasoning, lemon juice, and cayenne pepper (if using). Bring to a rolling boil over high heat.
Add the quartered red potatoes and onion to the boiling water. Cook for about 10-15 minutes, or until the potatoes are almost fork-tender.
Add the smoked andouille sausage (and kielbasa, if using) and the corn halves to the pot. Continue to boil for another 5-7 minutes.
Add the shrimp (and crab legs, if using) to the pot. Cook for just 3-5 minutes, or until the shrimp turn pink and opaque and the crab legs are heated through. Do not overcook the shrimp!
Carefully drain most of the liquid from the pot, leaving just enough to keep the ingredients moist. You can use a slotted spoon to remove the ingredients or carefully tilt the pot and drain.
Arrange the Low Country Boil directly onto a newspaper-covered table or large platters. Garnish with the halved lemons.