Believe it or not what you wear is almost as important as how and what you do in training. When selecting your race wear, err on the side of comfort. By this I mean the fit should allow you to breathe and move unrestricted and at the same time not flap and billow in the wind. Not only do you want to be comfortable but streamlined as well. Avoid things like surf trunks and cotton T-shirts.
Trunks create drag in the water and cotton gets heavy when wet and can cause chafing.
A top or singlet of some sort will keep you cooler because the moisture you produce takes longer to evaporate thus keeping you cool longer. Hydration starts well before the race starts. We in Hawaii are constantly in a battle against dehydration. Most people are unaware of how close to being dehydrated they are before they even start to work out. A good rule of thumb is to try to have clear urine all the time.