That's a bit of a problem of course. It's hard to say as the water levels do go up and down a lot. We were surprised how much they rose and fell.
We were on a bus almost every day - but just to take us from one site to another. There is a tremendous amount of walking involved when taking a River Cruise and you do have to pace yourself. Some of the trips are all day. We would leave at 9 am and not return til 5 pm.
But if I'm sounding unhappy - I'm not - I knew it would be like this and we thoroughly enjoyed every minute. Of course everything wasn't perfect - nothing ever is, but we will do it again.
All of the River cruiselines do qualify with the information about the possibility of having to be bussed from one destination to the next due to water level problems. I haven't heard of it happening very often, and I wouldn't worry about it too much. The accommodations they will give you if they must take you off the ship, will be first class. Even though it's a bit of a bother to pack an overnight bag, the staff is extremely helpful and will do everything they can to accommodate you.
I guess I would just consider it part of the adventure.