Bring water to a boil. Add shrimp; return to a boil. Lover heat, and simmer 3 to 5 minutes. Drain shrimp well; rinse with cold water. Chill. Peel and devein shrimp; set aside. Cut pineapple in half lengthwise. Scoop out pulp, leaving shells half to 1 cm thick. Cut pineapple pulp into bite-size pieces, discarding core. Set aside 1 cup pineapple pieces; reserve remaining pieces for use in other recipes. Peel, seed, and seciton orange and grapefruit; combine fruit sections with 1 cup pineapple pieces and shrimp. Cover; chill until ready to serve. Before serving, peel and dice avocado; sprinkle with lime juice to prevent discoloration. Combine avocado and shrimp mixture; spoon into reserved pineapple shells. Pour dressing over top before serving. Dressing: Combine all ingredients in container of electric blender; blend well. Serve immediately. Yield: 0.75 cup. SOURCE: Southern Living Magazine, July, 1980. Typed for you by Nancy Coleman.