NO irons means just that. It's not only a fire hazard, it's also the overloading of circuits.
The fact that you KNOW irons aren't allowed (or candles or heating pads) should be enough said. Why you would want to endanger yourself, as well as 2,000+ passengers, is beyond me.
All Carnival ships have a laundry room on every deck, and some of their ships have two laundry rooms on every deck. There are washers, dryers, ironing boards, and irons for passengers' convenience.
Just because some people 'get away with' having an iron in their cabin doesn't make it right or safe. Any cabin steward worth his salt will remove the iron from the cabin and return it to you prior to debarkation.
Think smart and think safe. Use Wrinkle Release on your clothing.
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