Do I need a passport for a cruise?

No, it is not a requirement if it is a closed loop cruise.  Meaning you depart and arrive at an American Port.

HOWEVER.  It is highly recommend that you have one, and I will tell you why.  Say a family member gets sick or passes away while you are on your cruise.  You CANNOT get home without a passport meaning you will be stuck until you come back to your US Port.  If anything happens to you and you become injured and need to come home…you need to have a passport to leave whatever country you are in at the time.  So while it is not required, it is definitely recommended.