frogfuzz,
Looking at an itenerary that has an overnight port stay, can you come and go from the ship 24 hours or do they have a "lock down"
They usually let you come and go around the clock, but there's not much open in the wee hours in many ports of call.
Of course, it's very easy to secure the gangway of a cruise ship -- and I have no doubt that all of the major cruise lines would do so in a heartbeat if a security threat or some other circumstance warrants such an action.
Norm.