I used to get this question often as a charter captain
...usually right before a guest stepped aboard the boat. Problem is that seasickness and its remedies are very individualized. Everyone responds to meds differently, so try out any med long before you leave shore and have to rely on it. Its mighty unpleasant to discover you have adverse reactions to a med while seasick too (fortunaltely Ive only observed this phenomena, not experienced it).
As suggested, Stergeron (sp?) is a popular choice with minimal side effects, but still a good idea to try it out first. Also be sure you work out the dosage correctly, which can be a challenge when crushing pills. There is a liquid version which is easier to measure, but its much less common. My GF got her dosages way wrong once last year and was out of commission for a full day...she wasn't seasick though! ;-)