There is a new boat called Allegra for people in wheelchair. I have not taken this cruise yet but plan in near future. The company that arranges this cruise is in UK and I believe it's called accessible travel. But will confirm more and post it's numbers
If one has OK mobility, i.e. can stand and take a couple of steps, it looks as if the Viking Longships might work. Their website says they will not take electric scooters, though. Unfortunately, the diagrams of the cabins do not show any wheelchair access. I'm still hoping for an accessible river cruise.