I've been cruising since I was 5 (I'm now 34) and can count on ONE HAND how many times I was sick on a ship. I can count using ONE FINGER how many times I was seasick.
You can pick up some OTC Dramamine, I would suggest the low- or non-drowsy one. DO NOT TAKE IT until you need it. Ask your pharmacist where you can find it.
I take it with me on every cruise. I was even on a boat cruise recently and took it. Guess who didn't need it, despite the Canal being rough? Most folks didn't take it....guess which ones almost regretted it!
It's always better to have it and not need it than to need it and not have it.
Also, get a stateroom on a low deck, midship, that will help A LOT!