I'll be taking a boat across the Strait of Magellan from Punta Arenas, Chile to Magdalena Island. I get moderate motion sickness (e.g., if I'm on a roller coaster with loops or I haven't ridden the subway in a while). I've been on a speedboat a couple of times over calm waters and haven't really felt anything. But the Strait of Magellan is supposed to get rough at times. I think it makes sense to come prepared. Just curious as to what works for you.