Preheat oven to 400 degrees. If using crab or lobster shells, use a rolling pin to break the shells into smaller pieces.
Place shells on a large foil-lined baking sheet and put the smashed garlic cloves on top. Roast the shells and garlic for 10 to 15 minutes, stirring halfway through.
Put roasted shells and garlic into a large stock pot and add remaining ingredients.
Simmer until broth is developed to your liking, adding more water as necessary, about 2 hours.