Inaccurate. For closed loop cruises out of the US as long as it is valid on the day your cruise ends there is no 6 month requirement. Cruising in other parts of the world may require six months before expiration in order to enter a specific country, but even that is not everywhere.
To the OP I would go ahead and renew it now, since it expires before your cruise. No need to expedite the service as it is currently being reported that renewals take 3 - 6 weeks.