Seasickness is classified as a type of motion sickness. It is an inconvenient condition that makes it difficult to enjoy boat rides and cruises. The motion of the sea (and how it affects the brain) is what makes the person feel ill.
Seasickness is caused when the sea goer loses his sense of spatial orientation. The movements of the ocean confuse the brain. The inner ear, eyes, and body parts send a signal to the brain that causes you to lose your sense of balance.
When someone gets seasick he may experience severe vomiting, disorientation and sleepiness.
Eating too much before a boat trip (especially spicy and fatty foods) could aggravate seasickness.
Spending too much time below deck, reading or staring at one point in the distance (such as the horizon) while on the boat for too long can trigger seasickness.
When someone says that you have gotten your "sea legs," it means that your brain has caught up with your body and has gotten used to the swaying.