It doesn't matter how you plan to bring it on, it's not allowed and will be confiscated.
https://www.carnival.com/help?topicid=1202
"Electrical and household appliances containing any kind of heating element, such as irons, clothes steamers, immersion heaters, heating blankets, water heaters, coffee machines, hot plates, toasters, heating pads, humidifier, etc. (All Carnival ships offer valet laundry service for a nominal fee, and most ships (excluding Carnival Firenze, Carnival Luminosa and Carnival Venezia) also provide facilities with ironing boards and irons.)"