Cuba is generally a very safe country, in fact it was named the Safest Country in the World at Madrid's International Tourism Fair. Violent crime is extremely rare in Cuba, so you can walk in most any neighborhood in Havana during the day and be quite safe. In the evening, traveling in groups is a good idea, and as in any large city, take precautions against theft: Pay attention to your surroundings and secure your camera gear, don’t leave your camera bag unattended while photographing, or leave it behind your chair during a meal.
Visitors should also be aware that medications are far less available in Cuba. Therefore, you should bring all prescription medications required for the entire trip, as well as any other medications or items (such as insulin, inhalers; epi-pens; etc.) advised by your physician. Over-the-counter medicines common in the U.S. are also not readily available in Cuba, even at pharmacies or clinics, so visitors are advised to bring a supply of ibuprofen or aspirin, Imodium or Pepto Bismol, hand sanitizer, and antacids if you may need them.