Have you tried the Scopolamine patch? My husband can get seasick from time to time if it’s rough, as well, and they work great for him. We were on a five day/four night nonstop sail from Puerto Rico to the Turks and Caicos many years ago with another couple helping them deliver their sailboat to Florida from Tortolla, BVI. The wife got so seasick that she was dead to the world. I put a patch on her (behind the ear) and within 15 minutes she was a new woman! Usually, you apply them before setting sail but she was not having a problem until the second or third day of rolling seas, four hour watches, hand steering because the autopilot was broken and staring at the wonky compass to hold course hundreds of miles from land. I can get a little queasy myself if the conditions are really bad, but I usually get over it with not much of a problem - it’s not fun, I know. I think you can still get the patches by prescription, though you can buy them over the counter in the BVIs, Turks and Caicos and, I think, the Bahamas.
In researching it, I read about an astronaut who had the same problem. He was fine until the splashdown, then he'd puke all over the capsule.
If I ever plan to cruise in the oceans, it'll have to be in one of those fast cigarette boats or maybe a seaplane! :)