Combine water, beer, Old Bay, salt, lemons, garlic, and onions in a large (8- to 10-quart) stockpot; bring to a boil over medium-high. Add potatoes, sausage, and corn, and cook until potatoes are tender, about 20 minutes. Using a slotted spoon; remove all solids from pot, and spread in a single layer on a large baking sheet lined with parchment paper or newspaper.

Return cooking liquid to a boil; add crabs, and cook until shells are bright orange and crabmeat flakes easily, about 10 minutes. Serve crabs with potatoes, sausage, corn, and lemon wedges.