So I get post cruise seasickness - mal debarquement. To prevent it I start taking meclizine the day before the cruise and throughout the cruise because in this case prevention is the cure.

Onboard ship, things that can help, always keep your stomach lightly full. Not huge heavy meals but anything to keep the stomach juices from rolling around. Bread, green apples, ginger can all help. And then there's my favorite, drinking an alcoholic drink (not enough to be drunk).