Having spent 27years in the Royal Navy, and spent weeks on end at sea in in a small frigate in the Iceland/Faroe Gap in horrendous seas, I feel I have something to say on the matter.
My first time at sea was crossing the channel in a minesweeper. I was 16years old and was a sick as a dog. Only "cure" was to go on the upper deck and keep an eye on the horizon, so it made my brain understand the movement. The problem on a ship, is that you can feel the movement, but unless you can see the outside stability, you get motion sickness.
After being ashore for months, then going back to sea, the seasickness would return, but after a day or two, you get used to it. Eventually, it disappears completely and you get to love the movement and delight in rough seas. Rougher the better.
Advice from me, is to get up top and look at the sea and not dwell on worrying about being ill. Try not to be in a place where you can't see outside. Try not to be all the way forward or aft, but aft is better than forward. (Aft is the blunt end, and Forward is the sharp end)
Good luck me hearties.
Mick F. Cornwall