In fact, the only times I can remember not being motion-sick even though I had good reason to were the few times I've been on boats.

Interesting! I get severe motion sickness, sometimes even when I'm driving (watching TV, scrolling down a screen or moving my head the wrong way can set it off, too). It got much worse in my early 20s, so I've never been game to get on a boat since. Maybe the motion is different enough to not aggravate the motion sickness?