EllenVannin cabins on the top deck roll further from side to side than those further down. The best cabins to have from the movement point of view are as low down in the ship as possible and in the middle.
I am prone to sea sickness, so avoid cruises, but we did do a winter Hurtigruten tour along the Norwegian coast several years ago. I missed the best Northern lights show because, despite sea sick tablets, I was lying on my bunk feeling very queasy.
I do the long ferry crossing from Portsmouth to Caen regularly and if I expect the sea to be rough, I take a seasick tablet as soon as I get on board and make sure we have a cabin. The seasick tablets, have something to eat and the tablets work like knock-out drops and I then sleep for the roughest part of the journey, from the time the ferry gets away from the protection of the Isle of Wight and before it has the protection of the Cotentin penninsula (the prevailing winds are westerlies).