What is it?
Seasickness happens when the body, inner ear, and eyes all send different signals to the brain, resulting in confusion and queasiness. It is a problem generally attributed to disturbance in the balance system of the inner ear (vestibular) system. Your sensory perception gets out of synch as these nerve fibers attempt to compensate for the unfamiliar motion of the ship moving through water.
The movement of a boat on a fluid sea creates stress in the portions of the brain responsible for balance. Perhaps that stress causes the brain to start malfunctioning as the land based environment it understands is suddenly not behaving as it should.
The visual stimulus is misleading as it reports things like cabin walls, and furniture, in such a way that the brain interprets these things as stable when they're not. Your brain is being told by the vision system that the world is stable, while the inner ear is screaming that it's not.
The good news for sufferers is that the condition often disappears without medical treatment within a few days. As your brain learns to compensate for the swaying and pitching of the boat you will get your “sea legs”.
One unfortunate aspect is that after a prolonged period at sea it may take a while for you to adjust to being on terra firma again.