In a large pot, combine water, beer, Cajun seasoning, and Old Bay seasoning. Add sliced onions, lemon halves, and garlic cloves. Bring to a vigorous boil for about 10 minutes.

Add potatoes to the boiling broth and cook uncovered for 10-15 minutes or until fork-tender.

Stir in corn and sausage, increase heat slightly, and continue boiling for an additional 5-7 minutes.

Gently add shrimp and snow crab clusters. Cook for 3-5 minutes until shrimp are pink and opaque.

In a separate pan, melt butter and sauté minced garlic until fragrant. Stir in lemon juice and parsley.

Drain the seafood boil, transfer to a serving platter, drizzle garlic butter sauce, and serve immediately.