It's bloody awful......I had a short spell on trawlers around 1970, as a radio operator, and we got thrown around all over the place. It wasn't too bad if we were fishing, but if we were dodging the weather, or running to or from the grounds off Norway it was bad. No so much seasick, as you get your sea legs, but no sleep, no hot food, difficult to wash or even go to the toilet, for days on end.
Funnily enough the only time I was really seasick was on a crossing from Hull to Zeebrugge on the old Norwave. There was a long swell running after a big blow, and it just caught me wrong, was sick all the way across.
When I worked for a major Norwegian Oil and Chemical company as Uk Shipping Manager, we had a captain of one of our LPG ships, who had been at sea for nearly 40 years, but he was seasick for the first 24 hours after any leave, regardless of the weather!!!