Depends...if you booked your excursion through Carnival, then you are guaranteed. However if you have a time already, my assumption is that you did not book it through Carnival. I'm hoping it's not in Cabo since you take a tender to get to shore and the wait could be up to an hour.
All ships will have announcements in the morning regarding debarkment. The trick is to pay attention to what is going on around the dock. I believe the announcement that general debarkment may begin does not air until the shore excursions through Carnival have already left. Do not wait for the announcement! Once you notice people leaving the ship, figure out where the point of debarkement is and get off. If you ask staff, they may tell you that you cannot leave until the general announcement, so you may have to do the detective work yourself.
Good luck!