Due to their proximity to the Canadian border, it's really easy to enter Canadian waters without knowing it. If you accidently swim into Canadian waters and are stopped by the Coast Guard, without the propper documents, you could have some serrious problems. It is advised that you use a map, GPS, or any other reliable navigation equiptment. Some islands are only a mile from the Canadian border, so it's very easy to cross over without even knowing it.
Be extremely careful where you go kayaking. If you don't have a lot of experience, it is advised that you take one of the kayak tours, or check with some local kayakers for the safest spots. Because of their location, there are several strong converging currents in several parts of the islands. When kayaking it is advised that you avoid the passes, and trying to kayak from island to island. Deception Pass between Whidbey and Fidalgo Island is said to be the most dangarous. Some of these areas have even been known to sweep away small boats. Several kayakers have been swept out by the currents. If you do decide to go kayaking, wear a life jacket, and a wetsuit if you're planning a longer trip.