Melt butter in a Dutch oven over low heat; add flour, stirring until smooth. Cook 1 minute, stirring constantly. Gradually add half-and-half; cook over medium heat, stirring constantly, until thickened. Add Cheddar cheese, salt, and pepper, stirring until cheese melts. Stir in onion and sherry.
Arrange shrimp, scallops, crab meat, artichokes, and water chestnuts in a lightly greased 3 1/2-quart shallow casserole. Pour sauce over seafood and vegetables; sprinkle with almonds and Parmesan cheese. Bake at 350° for 30 minutes or until lightly browned.