I was on a Carnival ship two years ago and it was awful. The food was terrible (the Americans and their french fries) and the smoke on board was as thick as pea soup.
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Food quality on board any line is a subjective type personal assessment. You'll find the majority of Carnival cruisers think the food is wonderful. As far as the smoke being as thick as pea soup, I disagree with that comment as it is an exaggeration. Carnival has limited smoking areas. The same is currently true of Holland America Line.