my wife and I went alone and it was pretty awkward during meals etc. You always have to sit with other people at meals, and after it became known on the boat that we were really the only non-drinkers it seemed like nobody wanted to really sit with us anymore. Nobody was mean about it, but we definitely felt out of place. I would suggest if you do a Viking cruise to definitely go with at least one other couple so that you can always sit together etc.
The cruise itself was amazing. Great food, great service and the visit to Normandy was a bucket list item. We will absolutely do more Viking river cruises but will go with other people.