Do I need a pport for a cruise from Tampa, to Mexico, then back to Tampa?

As long as you are coming back to the same port and you are a citizen of the USA, then no, you do not need a pport. Your cruise is considered a closed loop cruise. The reason they are highly recommended is if something happens while in a foreign port, then you would have great difficulty in that foreign port. It takes about 6 weeks to get a pport BTW.