Do you know the type of sea kayaks you will be using (recreational vs. touring)? If you will be using recreational kayaks (i.e., sit on top variety--these are what outfitters typically provide), then you will need to consider head to toe sun protection. The sun is fierce in Hawaii--don't underestimate it.
You definitely need a hat--one that covers your ears as it sucks to have the tops of your ears burned. Also, bear in mind that sunlight will be reflecting off the water, so you can get burned from the reflecting light (bottom of nose, chin, etc.). A good pair of sunglasses with polarized lenses is a must IMO. You may want to consider some light gloves for your hands, too.
Also, Kauai gets a lot of rain (depending on where you are, time of year, etc.)--there's a reason why it's the greenest of all of the Hawaiian islands.
I would recommend antibiotic ointment of some sort. Superficial wounds take longer to heal in that environment.
Water. Any naturally occuring fresh water down at sea level is going to be intestinal suicide. There are going to be wild pigs doing whatever wild pigs do in water further up the slope in the mountains. So, don't just filter the water. Nuke it with a robust water purification method.
I have to say that I am pretty jealous though. Despite the physically challenging environment, Kauai is beautiful. You may want to take an extra memory card (and battery) for your camera--it's a photogenic place. Oh, and don't forget a robust waterproofing case or pouch for your camera. Don't use a ziplock bag, but purchase an item geared for this purpose.