On the ferry to Denmark, when the North Sea was pretty choppy.
DFDS were handing out free super-strength seasickness tablets, as it was so bad and there were queues for the toilets and washbasins. Multiple people just throwing up all over the floor.
Top tip: if it’s a big boat, get onto as low a deck as you can and get in the middle of the boat. If it’s a small boat, get on deck and look at the horizon. Do not curl up, shut your eyes, and stick your head between your knees.