You can get sea sick while outside on an upper deck. Or even if you're looking out of the window of the land vehicle you're riding. As a kid, I got nausea on ~2h bus trips, even though I looked out the window. If you don't look out the window, the onset may be quicker and stronger, so there may be some mitigating effect.
Your best bet is medications, though.
Funny story.
This year I went to Ogasawara ("Bonin Islands"): 24 hours by boat. The sea was rough on the way there. Pretty strong winds which added several hours to the time. I puked like 20 times, the last 17 of those on completely empty stomach contents or just water.
On the way back I took the Japanese anti-nausea cocktail known as Aneron which contains four drugs, none of which are available in Canada over the counter. The instructions warn against combining with other meds. I disregarded that and also took Gravol for good measure. I slept almost the entire 24 hours (except for waking up once to reload on the shorter-acting Gravol), and walked like a zombie after alighting in Tokyo. My heart rate was down to 40 (and I was not in good aerobic shape, though getting there now).
