It all really depends, but with todays stabilizers on the newer ships, the ships starts to really rock at about 12 ft. swells. 20 ft. is really moving and 30 foot it gets hard to walk around. Usually around 4-6 ft (which is the normal, calm seas) someone with no seasickness will feel no motion at all.

On a modern, large cruise ship (over 100,000 gross tons), you will generally not feel 6ft waves in a significant way. For a massive vessel that is 1,000 feet long and nearly 200 feet high, a 6ft swell is relatively minor—akin to a car driving over a small speed bump. Modern ships are equipped with advanced stabilizers, which are large fins that extend from the side of the hull underwater to counteract the rolling motion caused by the sea. In 2026, computer-controlled systems can reduce a ship's roll by up to 90%. You might notice a very slight, rhythmic "vibration" or a gentle tilting if the ship is broadside to the waves, but it is rarely enough to cause seasickness for the average passenger. However, if the ship is smaller (like a luxury yacht or an expedition vessel) or if the waves are "choppy" rather than smooth swells, you may feel more movement in the higher decks or at the extreme front (bow) of the ship. Most travelers find that once they find their "sea legs" after the first few hours, 6ft seas go almost entirely unnoticed while they are dining, dancing, or sleeping.