I'm terrible with any kind of travelling - planes, boats, coaches, even some cars (unless I'm driving!), even when I go to IMAX, the only thing I seem to be OK on is a train! I went on a cruise and used anti-travel sickness tablets called Stugeron from the chemists. I know you said they make you drowsy but didn't know if you'd tried those ones.
And I fully expect people to start saying it's all in your mind, (wo)man up, etc, but I didn't expect these tablets to work at all, so not as if I was psychologically pinning my hopes on them, and they did work.
^^ What JB says is true too - looking at the horizon helps. I heard that it's a confusion in your brain when it can feel you're moving but can't actually see anything around you moving (such as when reading in a car or like you said, being in an inside cabin), which causes travel sickness. Dunno if that's boohickey but sounds like it makes sense to me.