Hi SarahL,
I hope you enjoy this tour - I went last October and the scenery is breathtaking (and so green)!
I have similar concerns over transport, and always make sure to take a travel sickness tablets before setting off and carry a travel pillow for the coach.
There are some winding roads which overlook ravines (as well as passing lush mountains) - you might feel more comfortable sitting in the aisle seat. I did so for the St. Moritz/ Chur excursion as it's a long drive: had an obstructed view of the scenery but it was a good opportunity for a short nap! The coach journeys are about 60 -90 mins.
The boat trip on Lake Constance is about 35 mins from Lindau to Bregenz: we went on a sunny day so it was steady but there was still a breeze. It's a small ferry/ catamaran with table seating - I moved to a forward-facing seat near the middle.
Skechers are fine: the excursions are often to towns and there are benches and cafes to stop and rest if you've had enough of walking around.
Have a great holiday, and don't forget to take a camera!