I went to school with a kid who'd spent three years on tankers running from Texas to New England. He showed slides from his camera mounted on the rail of the stern, pointing at the bow, in heavy seas. I couldn't believe the flexing.
This is a video of a Maersk ship in heavy seas. The first minute of outside is not as impressive as many videos I've seen, but starting just after one minute are shots of ship interiors which are scary.
