Excess sun causes sunburn and may lead to skin cancer. Keep you shirt on . You are cooler with a cotton t-shirt on than with a shirt off. As you sweat, your body cools you off. Wear something on your head, a bandana or a hat to keep the sun off.
Blisters are probably the most frequent medical problem a novice encounters is blisters.
Try not to grip the paddle too tight and change hand position periodically. Sometimes keeping your hands wet will help. Veteran racers call hut every ten strokes or so. At the call, paddlers switch sides. This keeps the paddlers fresher and lessens fatigue, especially with the bow paddler.