Marine motion is peculiar enough to have a disease named after its effect—seasickness. The occupation of seafaring exposes crew to the vagaries of the vessel dynamics as they ply in the watery medium. The waters of the world follow a universal dictum—irrespective of where the vessel plies, in freshwater or on the high seas, the movements affect human physiology in ways that need special attention. With the world comprising two-thirds of water and international trade largely dependent on shipping for the world economy to thrive, motion disorders in mariners warrent unique medical consideration. This chapter describes the effect of motion on the vestibular system of mariners, its implications ranging from benign discomfort to serious safety concerns, and its amelioration, both by therapeutic and by design-based preventive measures.