We did a similar cruise on Sea Princess a couple of years ago but in July. I packed layers so I could peel off if necessary!! It was 19 degs in Reykjavik much to our surprise!! Akureyri was cooler and Shetlands and Faeroes were quite damp and cloudy but not cold. The fjords were quite warm (I've looked at the photos and I had tshirt and cropped trousers on) I took micro fleece and cagoule and used them both. Hope this helps.