Although it was 8 years ago now, we were on an Avalon river cruise from Budapest to Bucharest. Due to low water the cruise ended a day early, and instead of a 2-hour bus trip from the Danube to Bucharest, we did a 9-hour bus trip, stopping at a restaurant on the way for lunch. So not ideal, but could have been worse.
(In response to request for people to post their lower Danube low-water experiences.....)