<P> There are several modes of diving based on the diving equipment used . </P> <P> The ability to dive and swim underwater while holding one's breath is considered a useful emergency skill, an important part of water sport and Navy safety training, and an enjoyable leisure activity . Underwater diving without breathing apparatus can be categorised as underwater swimming, snorkelling and freediving . These categories overlap considerably . Several competitive underwater sports are practised without breathing apparatus . </P> <P> Freediving precludes the use of external breathing devices, and relies on the ability of divers to hold their breath until resurfacing . The technique ranges from simple breath - hold diving to competitive apnea dives . Fins and a diving mask are often used in free diving to improve vision and provide more efficient propulsion . A short breathing tube called a snorkel allows the diver to breathe at the surface while the face is immersed . Snorkelling on the surface with no intention of diving is a popular water sport and recreational activity . </P> <P> Scuba diving is diving with a self - contained underwater breathing apparatus, which is completely independent of surface supply . Scuba gives the diver mobility and horizontal range far beyond the reach of an umbilical hose attached to surface - supplied diving equipment (SSDE). Scuba divers engaged in armed forces covert operations may be referred to as frogmen, combat divers or attack swimmers . </P>

What is the most important type of communication in the deep sea