If you get sea sick on a boat look at the horizon. It brings your vestibular system in sync with your visual input. If you look inside the boat your visual and vestibular systems are out of sync.

The same could have happened to you on the coach - the river gave you something distant to fix your eyes on because of the visual appeal, while other objects did not offer the same point of stability because they were not attractive enough for you to fixate of them or not distant enough to provide stability.