Remove meat from cooked lobster tail and claws, reserving shells. Chop meat; cover and place in refrigerator. Place lobster shells in a large zip-top plastic bag. Coarsely crush lobster shells using a meat mallet or rolling pin; set shells aside.
Peel shrimp, reserving shells. Cover and place shrimp in refrigerator.
Melt butter in a Dutch oven over medium-high heat. Add onion and celery; sauté for 2 minutes. Add reserved lobster shells, shrimp shells, and bay leaf; sauté 5 minutes or until shrimp shells turn orange. Slowly add 1/2 cup sherry, and cook 1 minute, stirring frequently. Add milk; bring mixture to a simmer. Reduce heat to medium; simmer 10 minutes, stirring occasionally (do not boil). Remove pan from heat. Cool milk mixture to room temperature; cover and chill overnight.
Strain milk mixture through a fine sieve into a bowl; discard solids. Set milk mixture aside.
Bring 1 1/2 cups water to a boil in a large saucepan over medium-high heat. Add potato; cook 5 minutes or until tender. Lightly spoon flour into a dry measuring cup; level with a knife. Combine flour and remaining 1/2 cup water, stirring well with a whisk. Add flour mixture to potato mixture; cook 1 minute or until thick, stirring occasionally. Stir in milk mixture; bring to a simmer. Stir in shrimp; cook 3 minutes or until shrimp are done. Remove from heat.
Pour one-third of the mixture in a blender, and process until smooth. Pour pureed mixture into a large bowl. Repeat procedure with remaining mixture in two batches. Return pureed mixture to pan. Stir in lobster meat and salt. Cook over medium heat 5 minutes or until thoroughly heated, stirring occasionally. Remove from heat; stir in cream and 1 teaspoon sherry. Ladle 1 1/3 cups stew into each of 6 bowls; sprinkle each serving with 1/2 teaspoon chopped parsley.