An octopus is one of the most recommended ways in which two divers can backup each other with air in case their own fails or they run out of it before they had expected. This way, an octopus and the buddy system allow divers to count on a second air source as long as their buddy is close enough and disposed to share their air, which not always happens.
There are some objections that are often done regarding the octopus and the buddy system. According to these objections, an octopus is not the best solution at the time of being in an emergency and running out of air, and divers should find other solutions instead of it.
Relying on an octopus and another person whom you might not even know while running out of air deep in the water is not always the best idea. The octopus and buddy system takes a good predisposition and relines from both parts which can assure that at the eventuality in which one of them runs out of air, the other will share his remaining air with the first one. If this doesn’t happen, that person could die before reaching the surface, so it is a really serious matter and those who use this system must know the other person and rely on each other.
According to statistics, many diving accidents occur when a diver runs out of air and tries to reach his buddy’s octopus unsuccessfully. Therefore, and according to some professional divers, using the buddy system and the octopus can be riskier than not using it. Once divers use the octopus based on the buddy system they might forget about checking their own air since they rely on their buddy and this leads to running out of it more often.
Another objection often said against the use of an octopus is that when one of the divers runs out of air and asks the second person to share theirs, if they started diving together then this second person will probably be also about to run out of it as well. Despite all this objections, the use of an octopus and the buddy system is still the backup system used by many divers and which can be very advantageous when used by two people who can trust and rely on each other.
Source: Dive Pilot