Mince the shallot, garlic, and horseradish. Add to a bowl with the apple cider vinegar, white vinegar, red pepper flakes, fresh parsley, and oregano. Season with salt to taste. Chill for at least thirty minutes.
Thoroughly scrub the shells (oysters, clams, and either cockles or mussels) under cold water until they are clean. For mussels, debeard them. Discard any shells that are open and do not close when tapped. Keep chilled until ready to cook.
Pour water into a shallow pan with a lid. Stir in the garlic broth base or seafood stock until dissolved. Season with additional salt to taste. Bring to a simmer.
Add the shells, flat side down for those applicable. Cover and cook for 3-6 minutes, until all the shells open. Discard any that do not open. Remove from the broth.
If using crayfish, add to the broth and boil for 4-5 minutes, until they are cooked through and bright red.
Pry the oysters apart (note: they will not open like clams and mussels do) and serve on half shells. Serve warm or chilled.
Gently loosen the meat from all the shells but leave it on the shell for serving. Spoon a teaspoon or two of the vinegar mixture onto each shell. Serve while contemplating the strategic moves of your favorite Game of Thrones characters. Enjoy!