Sea-sickness

Seasickness is a form of motion sickness characterized by a feeling of nausea and, in extreme cases, vertigo experienced after spending time on a craft on water. It is typically brought on by the rocking motion of the craft, but people who are particularly vulnerable to the condition can feel seasick simply by setting foot on a boat, even if the vessel is in dry dock.

Seasickness can be a debilitating condition and can be dangerous if the sufferer has an important role to carry out, such as steering a yacht through stormy seas while avoiding rocks and other hazards. It is also particularly hazardous for scuba divers who, through dehydration following vomiting, are at increased risk of decompression illness.

Cause
Human beings instinctively seek to remain upright by keeping their centre of gravity over their feet. The most important way this is achieved is by visual reference to surrounding objects, such as the horizon. Seasickness often results from the visual confusion on a moving craft, when nearby objects move with the motion of the craft. Because the lines of the masts, windows, and furniture on a ship are constantly shifting with respect to fixed references, humans can suffer a number of afflictions, especially those unaccustomed to being at sea.

Sea-sickness has such a remarkable effect because both the sense of sight and touch are disturbed by the motion of a craft on water. The severity of seasickness is also influenced by the irregular pressure of the bowels against the diaphragm as they shift with the rising and falling of the ship.

In his book The Human Body, Isaac Asimov related the anecdote about a seasick passenger whom a steward cheerfully assured that nobody ever died from seasickness. The passenger muttered, "Please--it's only the hope of dying that's keeping me alive."

Many experience similar effects while not at sea:
 * in railway carriages
 * in automobiles
 * on swings
 * while looking from a lofty precipice where known objects, being distant, are viewed under a new aspect and not so readily recognised
 * while walking on a wall or roof
 * while looking directly up to a roof
 * while observing the stars in the zenith
 * on walking into a round room, where there are no perpendicular lines of light and shade and the walls and ceiling are covered with an irregularly-spotted design
 * on twirling round, as in waltzing
 * while watching video captured by an unsteady camera

Prevention and remedy
Over-the-counter and prescription medications such as dramamine and scopolamine (as transdermal patches and tablets) are readily available. Ginger capsules are also considered effective in preventing motion sickness. Some sufferers find that wearing special wristbands helps stave off the condition.

Those suffering from seasickness who are unaccustomed to the motion of a ship often find relief by:
 * keeping their eyes directed to the fixed shore or horizon, where possible
 * lying down on their backs and closing their eyes
 * drinking any substance that is likely to temporarily diminish their senses of sight and touch
 * move into a position where fresh air is blowing on their face
 * sucking on crystallised ginger, sipping ginger tea or taking a capsule of ginger.
 * moving to the boat's center of gravity to eliminate motion due to pitch, roll and yaw (but not translation)
 * taking the helm of a yacht can reduce sickness as the sufferer has something to concentrate on, and can also anticipate the movement of the vessel

Unlike with a hangover, succumbing to nausea normally does not relieve the symptoms of seasickness, and, once started, is often difficult to stop.