The other motion sickness antidote that people sometimes forget is when on the boat to keep your eyes on the horizon as much as possible. It keeps your eyes and inner ear in agreement as to what movement is occurring, and thus limits motion sickness. I am fairly prone to motion sickness myself, and the one time I got motion sick on a snorkeling trip was in Costa Rica where we were on a boat with an upper sitting deck and being the fool I am I went to sit up there where the motion of the boat was amplified. You'll be fine.