The most hazardous creatures to divers are probably the divers themselves. A diver is exponentially more likely to injure himself through neglect of proper diving protocols, inadequate dive skills, or human error than he is to be attacked or injured by marine life. In fact, most aquatic life injuries are caused by the action of the diver.
Divers may purposefully or accidentally touch a dangerous creature, or provoke an attack by making an animal feel threatened. Unprovoked marine life attacks on scuba divers are exceptionally rare. As a general rule of thumb, give animals space and observe them respectfully and calmly from a distance. Never chase, touch, or corner a marine species. Don't harass the animals and they will not harass you.