For instance, diving shallower than 30 feet in calm. Yet such dive could be dangerous if the diver does not understand the consequences of breath-holding on ascent. Professional divers make these kinds of dives all the time. Of course, judging the danger of a dive is more a matter of probabilities than absolutes. We must also realize that there is no such thing as a completely safe dive.
Nobody knows all the physiological risks associated with diving.
A safe diver is one who is able to assess the risk factors accurately and has a sober knowledge that his or her ability is sufficient to overcome these risks. This obviously depends on the buddy. In many instances it would be safer to dive alone. For instance, many instructors would agree that it would be safer for them to be alone than with a student on a training dive. The one remaining piece of the puzzle is to determine how being alone, per se , affects the risk of a dive.