Okay, so this article
is from Australia but the same holds true in the U.S. and other countries. If you are posting a review of a ship, hotel, airline, car rental, etc., you should be careful and avoid taking great leaps in your allegations.
The linked article uses cockroaches as an example. Posting a review that says "the hotel was infested with roaches" when in reality you found one roach in your room is making a false statement. Any lawyer could make mincemeat of you, simply by asking if you were a licensed exterminator, if you inspected the entire hotel, etc. Same goes for a cruise line. When posting negatives, don't use sweeping terms and unsubstantiated allegations.
I recall a case from a couple years ago where a couple were banned for life from all RCCL cruise brands because of endless complaints they posted on "another" cruise forum.