A field guide to scuba diving — the gear that keeps you breathing, the physics that keeps you honest, the training that keeps you alive, and the hazards that demand respect. Scroll to descend.
Every rule in diving is downstream of three gas laws. Water is ~800× denser than air: every 10 m of seawater adds one full atmosphere of pressure. Your body, your air, and your dive computer are all negotiating with that fact.
*Approximate single-dive no-decompression limits on air, for illustration only. Always plan with your computer and tables.
Gas volume is inversely proportional to pressure. Governs equalization, buoyancy changes, lung overexpansion injuries, and why your BCD needs constant adjustment. The biggest proportional volume changes happen in the last 10 m near the surface.
Total pressure is the sum of each gas's partial pressure. At depth, the partial pressure of every gas you breathe multiplies — which is why oxygen becomes toxic deep, and why nitrox has depth limits.
Gas dissolves into liquids in proportion to its partial pressure. Your tissues quietly load nitrogen the whole dive. Ascend too fast and it comes back out as bubbles — the mechanism behind decompression sickness.
The foundation of neutral buoyancy — the defining skill of a good diver. Salt water is denser than fresh, so you need more weight in the ocean. Wetsuit compression at depth steals buoyancy, which your BCD gives back.
Scuba gear is a redundant life-support system you wear. Tap any item to expand it.
Certification cards ("C-cards") are issued by agencies, not governments, and are recognized worldwide. The instructor matters more than the logo — but each agency has a distinct culture. The typical path: Open Water → Advanced → Rescue → Divemaster / specialty and technical tracks.
| Agency | Origin | Known for | Character |
|---|---|---|---|
| PADIProfessional Association of Diving Instructors | USA, 1966 | The world's largest agency; certifies the majority of new divers | Highly standardized, modular courses; available almost anywhere on Earth |
| SSIScuba Schools International | USA, 1970 | Digital-first training, dive-center-centric model | Flexible structure; free digital materials; strong global footprint |
| NAUINational Association of Underwater Instructors | USA, 1960 | One of the oldest agencies; trains NASA's NBL divers | Instructor latitude; reputation for academic depth |
| CMASConfédération Mondiale des Activités Subaquatiques | France, 1959 | Co-founded by Jacques Cousteau; star system (1★/2★/3★) | Federation-based; strong in Europe and Latin America |
| BSACBritish Sub-Aqua Club | UK, 1953 | Club-based diving; the UK's national governing body | Thorough training tuned for cold, low-visibility water |
| GUEGlobal Underwater Explorers | USA, 1998 | "Doing It Right" standardized gear and team protocols | Demanding standards; cave and exploration pedigree |
| TDI / SDITechnical / Scuba Diving International | USA, 1994 | TDI is the largest technical-diving agency | Pragmatic; SDI handles recreational, TDI handles trimix, deco, rebreathers |
| RAIDRebreather Association of International Divers | 2007 | Born around rebreathers, now full recreational range | Fully online academics; emphasis on neutral-buoyancy skills |
Same ocean, very different dives. Each modality layers new skills, equipment, and risk management onto the basics.
Diving is statistically safe — comparable to driving — because divers train for these specific failure modes. Almost every one is preventable. Expand each hazard for the mechanism and the countermeasure.