Always carry water with you. It is a desert, and it is such a different heat than in Florida. The heat in Israel wicks away heat, so you don't realize how much you have sweat out.
Wear a hat and sunscreen is crucial. For a pale person like myself with a Jewish mother, wearing sunscreen is hammered into me from a young age. Every day I forgot to wear sunscreen, it showed.
Be careful. Besides being a desert, Israel is at war with many of its neighbors. Even if you are a tourist, you don't have to act like one, just educate yourself. If you aren't comfortable with the culture, then don't walk into the Arab shuk, or market. Be aware of your surroundings.