What is the lowest-impact regular¹ method to cross the Atlantic Ocean? Specifically, which method has the lowest carbon dioxide equivalent emission per passenger-kilometre? I'm aware there are other impacts to consider, but for this question I'm focussing on the carbon equivalent.
I thought a ship would be lower impact than a flight, but this appears to be false, at least when comparing to cruise ships. But what about freighters? They're slower, lack the luxuries of a cruise ship, and for freight there is little alternative. How does this affect the equation of the additional carbon footprint per passenger-kilometre?
See also: Impact of various travelling options for a more generic question on a similar topic.
¹By regular, I mean a way accessible for people without special skills or access to their own boats. Therefore, I'm not counting rowing, sailing or (until I can buy a ticket on one) solar boats.