Peel shrimp, and devein, if desired.
Bring 6 cups water and next 4 ingredients to a boil in a large saucepan; add shrimp. Cook 2 to 3 minutes or just until shrimp turn pink. Drain shrimp, reserving 1/2 cup liquid.
Stir together reserved liquid, ketchup, and next 3 ingredients. Stir in shrimp, tomato, and cilantro. Chill. Serve with avocado slices, lime wedges, and saltines, if desired.