I'm a quadriplegic with 6 months to live, and I just found out I have syringomyelia. Is it worth having surgery?

Generally not. Please see your md with the specifics for a better answer. An exam and detailed history is very important in your case.
Not recommended . Question is if surgery would improve your quality of life - this depends on the nature and completeness of your quadriplegia. I would only take the anesthesia risks and post-surgical pain if there was some hope that you gain significant mobility back. Maybe worthwhile discussing with your local providers.