Julie is stunned when her parents announce in mid-cruise that they're getting a divorce: a 13-year-old tomboy meets a boy and experiences the initial joy of love, and a pair of mature prudes fall madly in love while sharing the same cabin on the ABC Television Network's "The Love Boat," SATURDAY, SEPT. 23 (9:00-10:00 p.m., EDT). Norman Fell of ABC's "Three's Company" and Betty Garrett of ABC's "Laverne & Shirley" play cruise director Julie McCoy's father and mother in "Julie's Dilemma," written by Ben Joelson and Art Baer. James Coco is cast as an author of morally uplifting pamphlets and Dody Goodman as a prim censor who fall passionately in love during deck encounters but are unaware that they are cabinmates in "Who's Who" written by Bob Fraser and Rob Dames. Melissa Gilbert plays a young teen tomboy until she shares a tentative first kiss with an equally inexperienced 13-year-old pal played by Jimmy Baio of ABC's "Soap." Marla Adams and Edward Winter are seen as his parents and Ellen Travolta as the girl's mother in "Rocky," written by Ann Gibbs and Joel Kimmel.