I don't know. Ginger root is highly effective according to (sourced) wiki though. I agree that if sea travel was never a priority then there'd be no impetus to find a treatment. I'm sure humans somewhere would make sea travel (and hence a treatment for motion sickness) a priority.
Mmmm. I think we can assume that in this timeline, the human sensitivity to motion is so great that ginger is only a marginal help - it might let a seawitch stay in a boat for an extra ten minutes.