Dehydration can occur very quickly, so it's important to have a way to quickly rehydrate yourself. Consider carrying an extra bottle of water and include a packet of rehydration salts such as those made by CeraLyte or Adventure Medical Oral Rehydration Salts. If you find yourself suffering from dehydration when you're on the water, mix the salts with a liter of water to help replenish fluid loss.
Whistle or Air Horn
If you get into trouble when you're in the surf, you will need to have a whistle or air horn in your first aid kit so you can attract attention and get help.
Tendinitis can occur because of the repetitive motion of paddling. The best way to prevent tendinitis is through stretching. But include some pain relievers in your first aid kit along with a wrist support so you can safely get yourself back to shore if you do feel pain coming on caused by tendinitis.
Treat Cuts and Abrasions
Cuts and abrasions are one of the most common types of injuries a kayak surfer will have, particularly if the kayak capsizes and you find yourself getting cut up on nearby reef or scraping your skin on the bottom of the kayak. Those who have cut themselves on coral will know coral cuts are particularly painful and take longer to heal than regular cuts. Because of this, you will want to include in your kit some bandages, gauze pad, adhesive tape and iodine to immediately treat cuts and scrapes you suffer in the water. You will probably also want to make sure you have fresh water to clean the cut.
There's no use in having a well-equipped first aid kit if it gets wet the first time your kayak capsizes. Because of this, you will need a waterproof container. Consider getting a dry bag for your first aid kit and storing it in a hold. If you can't store it in a hold, tie off your dry bag with a bungee cord so you don't lose it in the surf.