You won't require passports in September of next year, but the clock is ticking towards the day when they will be needed - and not just to enter other countries, but to re-enter the U.S. (and yes, I mean U.S. citizens will require a passport to return to the U.S.). Actually, if you had something happen on your cruise which required you to fly home from Cozumel you would need a passport next year. The rules are different depending on how you are leaving and returning. Cruise ships get another year. Airline passengers do not. You can read the new rules here.
My advice? You have plenty of time. Get passports now. You are going to need them eventually.