Yes, you are right Walt, the mental expectation starts the 'rot', if you like, but to answer Bruce, it appears the real physical part is overload of the balance part of the brain from the motion/position sensors in the semicircular canals of the vestibular apparatus, or inner ear mechanism, (affected mainly by gravity, and the accelerations of motion), added to by a mismatch between what the eyes are telling the brain to expect, versus what it actually receives.
It for this reason that the best way to avoid this confusing overload, which just ends up getting the stomach so churned up it just "spits the dummy", is by cutting out as much confusing signal as possible. Hence the suggestion of wedging oneself in a position where you just ride the motion of the boat, which fits with what the eyes are preparing one for, if watching, but allows the muscles to relax, and not try to adjust position with every little movement, which they otherwise try to do to maintain balance. That is why the skipper sitting up like Jacky firmly fixed in the helm chair is less likely to experience it than anyone else on board. However, wedging oneself in a corner of the saloon, or wedging oneself in a bunk, so constant muscular compensation is not required has a similar effect. This is why so many people who are seasick just want to lie down - that is the body telling them what it wants to do to try and achieve the same result - a decrease in sensory overload. So-called fresh air and looking at the horizon, if not achieving this sensory reduction, just does not work.
The various anti-emetic medications mentioned by others work to some extent, by damping down the brain's reception of this sensory overload - but at the price of often making one drowsy. Certainly not good if you are the skipper.
Only known cure for seasickness "Sit under an apple tree"
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but this is what you wrote that I'm convinced was my problem.
