For instance, inside the cabin of a rocking boat, the inner ear detects changes in both up-and-down and side-to-side acceleration as one’s body bobs along with the boat.

But, since the cabin moves with the passenger, one’s eyes register a relatively stable scene. Agitated by this perceptual incongruity, the brain responds with a cascade of stress-related hormones that can ultimately lead to nausea, vomiting, and vertigo,

...the US NOAA explained as part of its Fast Facts series.

Additionally, an affected person’s symptoms can be magnified by the strong odors of things like diesel fumes and fish.

Seasickness usually occurs in the first 12 to 24 hours after “setting sail,” and dissipates once the body acclimates to the ship's motion.

It is rare for anyone to get or stay ill beyond the first couple of days at sea—unless the vessel encounters really rough waves.

Tips to help ease the symptoms of seasickness:

  • Maintain your fluid intake. Seasickness and related medications cause dehydration and headaches. Drink water, low-acidity juices like apple and carrot, or clear soup, and avoid milk and coffee.
  • Keep moving. Most people find that being busy keeps their minds off their discomfort.
  • Stay on deck, even if it’s raining, because the fresh air is often enough to speed recovery. The closed-in quarters below deck magnify the vessel’s motion and worsen symptoms.
  • Carry a plastic bag. This simple approach allows for peace of mind by eliminating some of the panic of getting seasick. If you have to vomit "over the side,” though, check the direction of the wind and waves. Staying leeward (the side of the ship that’s sheltered from the wind) will ensure that an unpleasant experience doesn’t get even worse.
  • Consider an over-the-counter medication to prevent or minimize motion sickness. A dose is usually recommended about an hour prior to setting sail, and as needed at sea. These medications tend to be dehydrating, though, so drink plenty of water.