In a large stockpot over med-high heat, combine the water and beer. Bring to a boil and add the cajun seasoning and Old Bay. Add your onion and lemon wedges. Stir to combine and let it boil for 15 minutes.
While your water is heating, prep your garlic butter sauce by melting the butter, garlic, Old Bay, Cajun seasoning and parsley in a small saucepan on the stove. Stir well until combined, about 5 minutes. Keep warm.
Add the andouille sausage and potatoes to your boiling water mixture, stirring well. Let cook for 10-15 min, until the potatoes are tender.
Add the corn, crab legs and shrimp. Continue boiling for 5 min, until the shrimp are pink.
Remove the seafood and veggies from the boil by straining out the water and place on newspaper or butcher paper in the middle of your table. You may also serve this on large baking sheets, if you prefer less mess. Pour the melted butter mixture over the top and serve! A nice loaf of sliced bread goes well on the side.