Layers! In New Zealand the answer is always layers! It can get quite cold and windy out on the deck of the boat. A glove and scarf are useful. We had a perfectly sunny day, however Milford Sound is one of the wettest places in the world so it's wise to take a waterproof jacket. Don't let the weather deter you though. You can just as easily sit inside the cabin of the boat and enjoy the view that way.