do you need a passport to get off the boat and visit Mexico and then re board?
Little late to be asking, doncha think?
A cruise is not a land vacation and currently the same rules do not apply as they do to land vacations ... currently ... so far. That is subject to change.
Because Mexico knows you're only going to be sullying their soil for a matter of 8-10 hours, they don't care if you have a passport; just spend your money, barf up your too-many margeritas, and leave. Land vacation? you need a passport to get into their country ... as well as prior permission.
On a cruise, the passport is for re-entering the United States. The US currently ... so far ... allows your birth certificate and drivers license to suffice.
SIG: If you're going to create the drama, at least supply the popcorn.