The Great Barrier Reef is generally very safe for solo female travelers. The locals and tour operators are friendly and respectful, and there are numerous organized activities and tours that are geared toward individual travelers. Since it's a popular destination, it well-policed and any potential safety concerns are more likely to be related to aquatic conditions than to violence or harassment. As with any place, it's advisable to use common sense and observe basic safety measures such as not leaving your belongings unattended.