It is wise idea to come with everything you need to be warm and dry so you are prepared no matter what the weather decides to do. This includes a rain jacket, dry clothes to change into, just in case you do get wet, a warm synthetic layer or fleece, and a pair of shoes or sandals you can get wet. You will get wet up to your ankles or knees while getting in and out of the kayaks. Cotton is probably the worst thing one can wear when wet, as cotton wicks heat away from the body and does not dry quickly.
Don’t forget your sunglasses, sunscreen and maybe a hat. When you’re in the San Juans in the summer, chances are you’ll see the sun.