I am amazed at how complicated folks want to make this.
1 - Passport is not needed on closed loop cruises as per the U. S. Government.
2 - Virgin has been big on requiring it, but has loosed up a bit if you have govt ID and birth cert.
3 - If your ONLY id is going to be a passport, then the 6 month rule kicks in.
4 - What any other cruiseline says or does means close to nothing.
5 - Virgin has their requirements right on the website.
6 - And yes, there are good reason to take a passport, even if not required.
And please not get into the argument about leaving it on the ship or taking it with you. That argument is as old as a few other things that will not be resolved in our lifetime.