Dress in layers so you can change throughout the day as the weather changes
Multiple thin layers are better than one or two thick layers
Moisture-wicking/quick-drying fabrics are most effective in keeping you warm and dry
Make sure rain gear is made of waterproof fabric with waterproof taped seams
Waterproof rubber boots and/or hiking boots are recommended (we didn't take rubber boots)

Moisture-wicking long & short sleeve shirts (polypropylene, merino wool, etc.)
Full-length or convertible hiking pants
Sweaters, sweatshirts, long-sleeve fleece
Casual t-shirts, jeans/khakis, shirts/shorts
Socks (synthetic or wool) for all activities (We like Smartwool brand)
Bathing suit
Waterproof rain jacket and rain pants
Insulated jacket and/or vest
Wool beanie, scarf, ballcap
Waterproof gloves and kayaking gloves (pogies)
Calf-to-knee rubber boots (if you have them) for wet landings
Waterproof hiking boots with tread
Comfortable walking shoes for onboard
Water bottle
Lightweight daypack
Drybag for wallet, cell phone, camera, etc.
Backpack rain cover
Sunglasses
Sunscreen
Umbrella - We like Davek Umbrellas. (They are pricey but really good quality and last a long time. Davek also has a great warranty program. Get 'em when they're on sale.)