John,

Do you get carsick? If not, you are probably not prone to motion
sickness. I got sick on a bus trip once (only about a 45 minute trip). I
came to realize that it was due to a combination of factors. Firstly, I was
slightly dehydrated. This is a Big contributor to motion sickness.
Secondly, I sat near the rear of the bus and it is the rear of the bus that
moves the most, due to the solid rear axle.
I believe, also, that a contributing factor is the fact that it is quite
possible to feel claustrophobic on a bus, due to sitting in a long, narrow
box with typically one entrance/exit at the extreme end of the box.
Compounding that is the fact that you are not at the wheel.
Chances are very good that at the helm of your own boat out on the open
water you'll be okay. Especially once you get used to it.
You may want to acclimatize yourself to waterborne travel by kayaking
for a while.

Good luck,
Bill Longyard