Some of the Channel ferries are bigger than they were post-war, but they are still famous for rough crossings. Like "night trains" (for those who can't afford a hotel) on this newsboard, they are an obsolete hangover from a past era, barely hanging on.
I was sick on the ferry from New London, Connecticut to Orient Point, Long Island. But it was the dead of winter, the winter boat can only carry six cars (i.e. it's tiny), and my car was covered with salt spray when I drove it off. I wasn't the only one. My wife was sick on the Channel ferry around 1977.
My other point was that you still have to get from Portsmouth to your real destination. I didn't do the research for you, but it's most unlikely that the same Paris RR station serves Caen and London!