Add water, beer, Old Bay seasoning, and salt to a large slow cooker. Stir to combine.
Add potatoes and onion. Cover and cook on high for 3 hours or until potatoes are just tender.
Add sausage and corn. Cover and cook for another 1 hour.
Add shrimp and cook for an additional 15-20 minutes, or until shrimp are pink and cooked through.
Drain liquid, then drizzle melted butter over the top and toss gently.
Garnish with parsley and serve with lemon wedges.