Add water, vinegar, garlic, and Old Bay to a large stockpot.
Boil over high heat.
When the water reaches a rolling boil, add the shrimp and stir.
After approximately 3 min the shrimp should be done. When they are done they are completely opaque, have a pink/peach color to them, and the tails are curled up.
As soon as they look done, pour them into a strainer to drain off the water.
Now you can either serve immediately OR dunk them into an ice bath to stop the cooking and chill if you are serving as a shrimp cocktail appetizer.
Cocktail Sauce
Mix all ingredients together in a small bowl until combined.