Seasickness is a common phenomenon on board seagoing ships, and it may affect a significant percentage of crew and passengers, particularly in heavy weather and when sailing in high seas. It may be regarded as the prototypic and most widely recognized form of motion sickness and it has been a nemesis of seafaring ever since man first dared to take out to sea in dugout canoes. Up to date seasickness has been understood as a potentially serious medical condition and people affected are rather mocked than pitied, since it is reversible and people recover quickly. Still, it remains a treacherous illness with seasickness has treacherous when seriously impacting on the mental and physical performance of professionals or when progressing into a crisis after sustained vomiting and fluid loss. This may imply social, technical and procedural consequences, in particular relating to the safety of navigation. In today´s shipping crew headcount is decreased to the minimum required and the (functional) loss of a significant number of crew due to seasickness may well impact on the navigation and overall safety of a ship. This holds true for yachtsmen as well, who in contrast to professional sailors often lack routine, regular training and exposure to heavy weather. In severe weather conditions all hands on small vessel may be needed, however a significant proportion may simply not be able to leave their bunks due to seasickness. The additional strain put on the remaining active crew puts them at risk of getting tired, which in turn renders them more susceptible to make errors as well as to seasickness. Apart from these navigational and safety related implications, seasickness may in some cases also have serious medical relevance, for instance if psychosis develops and the affected individual tries to jump over board, or if a diabetic patient becomes hypoglycemic. Motion sickness may also aggravate loss of body heat in immersion, thereby promoting hypothermia and reducing survival times in naval accidents .

Note: Section 20.6 Treatment of seasickness is currently pending an update and review.