This depends entirely on where you are diving and what you understand by “dangerous”. In some areas, there are corals or plants which may sting if you brush up against them. Most marine animals do not attack and have stingers for defensive reasons only. Your instructor will explain to you how to interact with marine life in an observatory way which means you will experience some great encounters with many wonderful species. The golden rule is not to touch anything – most humans would be annoyed if something they didn’t know came into their environment and poked them!