An alternative evil scenario is as follows: many of the cruises book people on the same flights (or have charters) to get them from Europe to the embarkation point. Let's assume that one of these planes is leaving from Heathrow. A terrorist gets on a domestic flight in the UK that disembarks in Heathrow an hour before the "cruise" flight leaves. The terrorist then goes to the boarding area and spreads some aerosol very discreetly. These folks then climb on a plane and sit together closely as the air ciruculates and the virus spreads. By the time they land in Miami there are a bunch of carriers who all then climb on a boat and further spread the infection. At the end of the cruise the boat load then disembark and return to points all over the western world where they further spread the virus.
In this scenario the Terrorist doesn't have to actually smuggle anything into the US or approach a boat that is being heavily watched.
A science fiction author once wrote "Coincidence means you weren't paying attention to the other half of what was going on."
-PJ