All ships/cruise lines are a little different. Here is what Viking says on their site:
What Is The Dress Code?
Dress during the day is casual including shorts (if the season is warm), pants or jeans and comfortable shoes for walking tours. Swimsuits, brief shorts, cover-ups and exercise attire should be reserved for the Fitness Center, pool areas and Sports Deck. There are no “formal nights” in the evening; evening dress is “elegant casual” for all dining venues after 6:00 PM, performances and special events. On these occasions, required attire for ladies includes a dress, skirt or slacks with a sweater or blouse; for gentlemen, trousers and a collared shirt. A tie and jacket are optional.
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Only other thing I would add is that it is Holy Week when we are there so if any excursions involve any religious sites you might want to et on a bit more dressed up/neat/respectful