Combine egg yolks, vinegar, flour, and butter in a medium saucepan; stir well. Cook over medium heat, stirring constantly, 10 minutes or until sauce is thickened. Cool completely. Chill.
Place oysters and water in a medium saucepan; cover and cook over medium heat 5 minutes. Drain and chop oysters.
Combine cracker crumbs, pepper, and celery seeds; mix well and add to chopped oysters. Stir slightly to coat oysters. Add half of chilled dressing to oyster mixture; toss gently until all ingredients are moistened. Spoon oysters onto a bed of lettuce, and pour remaining dressing over top.
Note: This dish can also be served as an hors d'oeuvre.