In all the modes of travel I’ve been on , plane , train or ship, the smoothest by far was traveling at depth in a submarine. You had no more feeling of motion than we have of the Earth spinning or going around the sun.
angles and dangles is a hoot
We usually patrolled at around 200 feet and 95% of the time it felt motionless.
Very rarely I felt the topside waves down to around 400 feet if they were bad enough.