When I was sailing, both as a cadet and afterworn, I never got seasick. This includes ships of just about every shape and size, from ocean tugs to Panamax containerships. The only time I have ever been seasick was once I came "ashore". I was out, offshore on a drilling rig doing surveys and caught a ride back in onboard a small diving boat. It was a short run (about an hour and a half), but the motion just tore me up. It never happened before, and hasn't happened since. These days, most of my boat rides are on crew boats, dive boats, ROV boats, etc. But that one ride made me understand how those that DO get seasick feel. Of course, it could also have been the night lunch on the rig, too.