As a person that has spent lots of time on the ocean (Pacific & Atlantic) if you are going to be on a Cargo Ship, You don't need to worry about getting sea sick. It is a CARGO SHIP!! Those things are HUGE and not that bad a ride.
Just remember "if" sick DO NOT go lay in a rack inside the skin of the ship!!!! That is the absolute worst thing to do!! Get your BUTT outside for 2 reasons. 1st it will help you & 2nd if you puke inside the ship they might just throw you overboard b/c that crap stinks up the entire ship!!!
If you are this worried, maybe you should just stay on land.