In a small saucepan, combine the sugar, cornstarch and salt; stir in vinegar and water until smooth. Cook over medium heat, stirring constantly, until mixture comes to a boil. Cook and stir for 2 minutes or until thickened. Cool completely.
In a large bowl, combine with cucumbers, celery, shrimp, scallops and sesame seeds. Add dressing; stir gently to coat. Refrigerate for 2 hours. Stir before serving.