Thank you for all the comments and recommendations.
For clarification, (I guess I didn't think about writing that info when I first wrote my post), I am a US Citizen living in the US.
When I had entered my passport info and the expiration date of the passport during check-in, the website "highly recommended" that my passport expire more than 6 months from the travel date.
After reviewing Celebrity's documentation requirements and options, I'm going to use my US Naturalization certificate and state driver's license. I'm so use to using my passport for traveling I didn't know I could use other forms of ID for a closed loop cruise.