Then stay cognizant of not sitting where you watch the horizon bobbing, stay looking inside the boat.
That may be a personal thing, as it's NOT what would work for me. My understanding is that seasickness (and motion sickness in general) is caused when the brain receives what it determines to be conflicting signals from the eyes and inner ear. Looking inside the boat will result in being stationary as far as the eyes are concerned and moving as far as the inner ear is concerned. If you're prone to it, that's probably a recipe for being sick. If you do sit and watch the horizon, don't watch it bob; if you move your head to follow the horizon, the motion will make more sense to the brain. Why the brain figures nausea and vomiting will solve the problem, though, is beyond me.
Either way, if you're prone to seasickness or find out on the boat that you are, find out what ways of dealing with it (meds, watching or not watching the horizon, etc) work for you and take note of what will provoke it or make it worse and avoid those if you can.