The item that is overlooked the most is clothing. Too many times I have been out on water that is dangerously cold and have passed fellow paddlers wearing nothing more than a cotton t-shirt and shorts. Regardless of what that air temperature is, you need to dress for the temperature of the water. The waters in Northern New England are cold even during the warmest of summers days. Hypothermia is the greatest threat to one's life during a kayaking accident and it only takes a few minutes for you to start feeling its effects.
There are a couple of sayings out there and both of them are true; "cotton is rotten" and "cotton kills". When cotton becomes wet it is slow to dry and is an excellent conductor of heat. This is the last thing you need if you find yourself in cold water. To put it in just a few words; stay away from cotton. There is a huge selection of paddling wear out there, made from a variety of synthetic materials. During the early spring and late fall (or during the winter for those brave enough to venture out) the best protection against cold water is a dry suit. These are one-piece, waterproof suits, that usually cost anywhere from $600 to $900. If you plan on spending any amount of time out in the colder weather it can be a lifesaving investment. The next down the list would be a two-piece dry suit; these are less effective and are not 100% waterproof. They will provide enough protection to allow yourself to get out of the water and back into your kayak if the need arises. They will also work very well as a rain suit. The tops and bottoms are usually purchased separately and will set you back between $250 and $350 for both pieces. Another alternative to the dry suit is the wet suit. They too, provide excellent protection against the cold water, but can become uncomfortable to paddle in if worn for any length of time. After the waters have warmed some, NRS (Northwest River Supply) has a good line of paddling shirts and bottoms appropriately called Hydroskins. They are manufactured from a lined neoprene, are very comfortable and also make an excellent base layer under a dry suit.
Don't forget your head, hands and feet. Again, there is a huge selection to choose from out on the market, so shop around and choose the gear that is comfortable to you. I do recommend that you stay away from wearing sandals while kayaking during the summer. The soles tend to be too stiff, and you will find yourself constantly cleaning the dirt and sand from out between the soles of our feet and the sandals. Instead of sandals, try a pair of water shoes. They are usually made from neoprene and have soft, flexible sole that is made from a sticky rubber which will provide excellent traction on those wet rocks. When it comes to gloves, the two pair that I use most often are a full fingered neoprene glove from Warmers for cold weather paddling and a fingerless pair by NRS for warm weather paddling. Two more items I would like to mention are sunglasses and hats. Make sure the sunglasses you choose are polarized; these will greatly improve your vision out on the water. Since most of your body heat is lost though your head, don't forget to cover it up. Warmers manufacture a fleece lined, neoprene cap that is excellent for cold water paddling.