Neve,
My eldest brother is an ENT based in England, and while he doesn't dive, he says the most common and basic (excluding getting a massive hang-over from the previous night) reason for motion sickness while on a boat, is not getting sufficient sleep the night before.
I find that he's right on this point (for once; heheheh). When I recall my old deep sea fishing days, I now realize that I get seasick much easier if I haven't slept well the previous night, for whatever reason. Taking dramamine or whatever doesn't help much.
BTW, the patches are a bit hard to come by nowadays as reported by a pharmacy, and they're on prescription as well. Wonder why. But they said I could try some other large pharmacy chains to be sure, which I'll do tomorrow.