Mexican Shrimp Cocktail

2 pounds unpeeled, large fresh shrimp
6 cups water
2 to 3 tablespoons fresh lime juice
3 garlic cloves, pressed
1/4 teaspoon salt
1/2 teaspoon pepper
1 cup ketchup
1/2 cup fresh lime juice
1/4 cup minced sweet onion
1/2 to 1 teaspoon hot sauce
1/2 cup chopped tomato
1/2 cup chopped fresh cilantro
1 ripe avocado - peeled, pitted and chopped