Combine water, beer, powdered stock, garlic, butter and half of the parsley in a large stock pot with a lid and bring to a strong boil. Toss in clams and close lid for 7-10 minutes until clams have given up the fight and most are opened. Add the mussels, recover and reduce heat to low. Keep covered for 3-5 minutes and then check for any unopened mussels. Sprinkle remaining parsley over shellfish and serve family-style or in individual bowls with French or sourdough bread for dipping in the broth.