Quick Note: Comfort
While reading a thesis about tilting trains, I saw a comparison of passenger comfort on different modes of transportation. This includes the following graph (p. 30), which the thesis sources to a study of motion sickness in US children and teenagers:
The scale is originally 0-3: this study polled a sample aged 9-18 and asked whether they feel nauseous on any of the above modes, where 0 is “never” and 3 “always.”
Pedestrian Observations 
Fits my anecdotal evidence. I get motion sick really easily. I get motion sick in everything in that graph except bicycles, escalators, elevators, swings, and trains.
And I’m male, so that makes me absolutely typical. I can’t imagine what the gender difference for elevators is about; most of the others just seem to be part of “women are more likely to get motion sick period”, but the elevator one is quite extraordinary.
Well, I can’t speak for teenage boys, Nathanael, but if an elevator really drops I feel it in my testicles. Apparently that doesn’t happen for teenage girls.
I think part of the difference on results come from the fact certain vehicles have a very uniform, limited use like escalators, whereas cars, for instance, provide a different experience in a fast-flowing freeway or twisting mountain road to some sky resort.
It happens if there are weird turning angles, unnatural acceleration, and the brain can’t synchronize the motion with visual expectations. Thus, sea sickness, roller coaster/airplane barfing, and backseat nausea in cars. The more ‘natural’, i.e. in tune with human physiology/evolution, the transportation mode the less motion sickness -> escalators, bicycling. Walking is pretty good in that regard, too.
The worst: riding a bus that drives up a winding road in a backward seat.
Those proportions sound about right to me too. I wish they had also distinguished between subways, standard passenger rail, and streetcars, each of which has its own set of triggers (fast acceleration and deceleration and poor visibility for subways, side-to-side rocking and diesel fumes for passenger rail, jerky stopping and starting for streetcars).
“Cruise ships” is way too broad. If youre on a 300 person ship, youre going to feel the rolling more than on a 4,000 person boat.