As the Scots are fond of saying, “If you like all four seasons, you’ll love a day in Scotland.” This is not an overstatement, as conditions can vary dramatically in the course of a few minutes from hot sun to pelting hailstorm.
For a tour of western Scotland, including the shore excursions from the Hebridean Princess, I recommend the following: sturdy waterproof shoes for walking; a lightweight, waterproof jacket with hood (both L.L. Bean and Lands’ End make good ones); lightweight “adventure” pants that are quick-drying; a light microfleece pullover such as those made by The North Face or Patagonia; quick-dry socks; lightweight fleece vest; a broad-brimmed, foldable hat (Columbia has a nice one); and fleece gloves.
With these you can add or remove as the changing weather dictates. You might also pack a small collapsible umbrella — more useful in towns and cities than in the countryside. I would also highly recommend a good pair of small binoculars, and if you are going to drive, I found the Michelin map of Scotland (No. 501) to be perfectly reliable.