We went with Hurtigruten to Norway, flying to Bergen and did the South to North route in January to see (and did see) the Northern Lights. The ships are very comfortable although I would advise upgrading at least one level. They have thick hulls because of the ice so the lower cabin portholes are more like two foot deep tubes! I rarely noticed the ship roll at all as they're modern and stayed in sheltered waters between the islands. There are lifts between floors and good buffet meals. No dressing for dinner or Captains table. The ships are also postal and ferry ships so they stay close into the coast often only stopping for fifteen minutes to drop off and pick up so you see a lot of the coast and small towns as they call in along the way. There are maps by the gangway of the ports where they stay longer and excursions and daily talks. Coaches meet the ship as it docks and time it perfectly for dropping off back at the bottom of the gangway. There are walking tours around the towns too, but since maps are provided of each town as you dock so you can easily do your own thing. I was walking with two sticks at the time and got around fine with help always there when I needed it.
We also did a Nile cruise with Saga. More expensive than most but they park their boats in better spots, such as right next to the temples in Luxor rather than a couple of miles outside the town. One lady on board, in her nineties, never got off the boat, sitting on the top deck in the sun and watching Egypt float serenely by. Several temples would be fairly disabled accessible though, and being Saga they were very aware of the problems of older people. The ship had its own water filtration set up and you could visit the kitchens if you wished to prove how clean they were! This trip was before Saga was taken over so may be different now.