Cruise.com via ShermansCruise offers a Carnival Cruise 7-Night Eastern Caribbean Cruise for Two with prices starting at $778. (On the ShermansCruise landing page, scroll to the bottom and click "Book Now" next to the December 9 sailing date.) That's the lowest price we could find for such a cruise in December by $20. This roundtrip cruise aboard the Carnival Fascination departs from San Juan, Puerto Rico, on December 9. Book this deal by September 30.