This cruise company cater for trips to the Fjords and you can wear what you want. Formal dining is optional only if you want to take part, if not choose to dine in the other areas of the ship. T-shirts, shorts and tracksuits are not allowed in the restaurants for evening dining.
http://www.fredolsencruises.com
For more info click on the Tab for 'Before You Cruise' and then 'Dress Code'.