It varies. I used to FEEL sick all the time in the car but never was actually sick. I don't really feel sick at all in cars now unless I look down. I wouldn't be able to use my phone in the car.
I always feel sick on flights but again never was until this July which I think was brought on by having a bad cough/cold.
I've been on ferries a few times and was never ill but I stayed on deck the whole time. I felt a bit sick on the cruise I did in July but mainly on the one day the weather was bad and the sea was rough. I wore sea bands the whole time (visible in ALL the photographs!!) and took about 4 travel sickness tablets over the week which wasn't too bad. I actually found lying flat on my back really took the sensation away which was a relief. Green apple is also meant to help. Everyone recommended ginger to me so I bought some ginger sweets but they tasted awful and I didn't bother with them.
Don't quote me on this but I think motion sickness is caused when the vehicle moves but what you see appears to be still (either because you're looking down or can't see a fixed point outside such as the horizon or just normal outside objects) and the balance sensors in your brain don't match up because they think you should be completely still but you aren't. Something like that.