These things are usually not a big deal and blow over in a couple days. However, if things need to be revised the cruise line will e-mail you and/or your agent if there is enough time before your cruise starts. If port safety becomes a problem during your cruise the ship's staff will inform you as soon as they can, usually by written notification delivered to your cabin. The Captain will also likely do a public announcement if the itinerary has to be changed.
They will not risk endangering the passengers, crew, and ship, in that order.
Please don't let this dwell on your mind. It is what it is. The cruise line will have no control over things. In my experience it will likely blow over and you'll be able to see Athens.