The depth you can you dive with an open water certification with diving organisations like PADI and SSI is 18 metres (60 feet). But there are other diving organisations that also call their first certification level ‘open water diver.’ Some use the 18 metres depth level, whereas others use 20 metres (66 feet) as their safe depth for newly certified scuba divers. BSAC call their first certification level Ocean Diver, where the depth limit is set at 20 metres.
The common depths of beginner scuba divers is somewhere between 18-20 metres (59-66 feet). This limit is set due to the pressure of water at depth and the implications deep water has on scuba divers. Why this is important is due to oxygen toxicity; Boyle’s Law and how the changing pressures at depth impacts gas volumes; decompression sickness or the bends; and nitrogen narcosis. At the next stage of learning to scuba dive, the most common depths are between 18 and 35 metres (59-115 feet).