with BobS
The passport will eventually (and soon) be mandatory anytime you leave the U.S., no matter whether by air, sea, car, foot or on your hands and knees. It is a very valuable document to have, and well worth what sounds like an exorbitant amount of money. But, it's good for 10 years.
And, you must be careful in that "leaving from and returning to the same port" phrase, because it absolutely does not apply when your ship docks at any foreign point between sailing and returning. For example, both Puerto Rico and the U.S. Virgin Islands are essentailly U.S. entities. But almost any cruise ship sailing from Florida will stop at the cruise line's private resort - which are mostly in the Bahamas, a foreign country. You may need to show your passport at San Juan, Charlotte Amalie, etc., because you have been to a foreign destination before arriving in a U.S. one.
Hope that isn't too confusing.
SWR
