From personal experience, I know that even if you go out onto the deck and look out on the ocean, so that you have a visual reference to the horizon, it doesn't help.
You can look out of the window of the car and still get motion sickness; it doesn't only happen to people who are engaged with something inside the vehicle.
The external visual reference doesn't entirely help the fact that your body is experiencing accelerations not of its own accord.
The leading hypothesis on motion sickness is that your brain interprets the mismatched motion sensations as being caused by a poison acting on your brain, and triggers a response to empty your stomach contents and make you averse to more ingestion.
This mechanism is not easily defeated by visual tricks. If they can slow the onset and reduce the severity, that is welcome, of course.
BTW, has anyone here tried the motion sickness glasses? This is an object which has a hollow rim, partially filled with colored fluid. The glass rims, as well as additional circles placed laterally near the temples, act as "semicircular canals": the fluid moves to provide a kind of horizon reference.