How a first course fits together
Almost every entry-level scuba course, whatever the agency, has the same three parts. You learn the theory, practice skills in a pool or calm shallow water, then repeat those skills on a set of training dives in open water. The names and details vary. The shape stays the same.
Part 1
Theory
Online, a book or a classroom
How pressure affects your body and your air, how gear works, how to plan a dive and handle problems.
Ends with quizzes and a final exam
More on theoryPart 2
Pool
A pool or calm, shallow water
Your first breaths underwater, then skills such as clearing a flooded mask and sharing air, repeated until they feel routine.
Ends when every skill is done to standard
More on poolPart 3
Open water
A lake, quarry or the sea
Usually about four dives over at least two days, where you repeat the pool skills in real conditions with an instructor.
Ends with your certification card
More on open water
The full picture, including what you need before day one and what your card lets you do afterward, is on how certification courses are structured.
Depth, pressure and why training has limits
Water is heavy. Every 10 m (33 ft) of seawater adds about the same pressure as the whole atmosphere above it, and the air you breathe squeezes to match. The gauge below is drawn from that one rule. It shows why your ears need equalizing on the way down, why a tank empties faster at depth and why courses set depth limits for new divers.
- 0 m0 ft1.0 barair at 100%SurfaceEvery dive starts and ends here. Air in your lungs, mask and BCD is at full size.
- 3 m10 ft1.3 barair at 77%Pool and confined waterAbout as deep as first skills go: a pool or a calm, shallow spot.
- 5 m15 ft1.5 barair at 67%Safety stopMany courses teach a pause of about 3 minutes near here on the way up.
- 10 m33 ft2.0 barair at 50%Pressure has doubledAir takes half the space it did at the surface, so a tank empties about twice as fast.
- 18 m60 ft2.8 barair at 36%Entry-level depth, oftenMany first certifications advise staying above about 18 to 20 m. Your own card sets your limit.
- 30 m100 ft4.0 barair at 25%Further trainingLater courses often take divers toward about 30 m, with more planning and supervision.
- 40 m130 ft5.0 barair at 20%Recreational limitAbout the deepest point recreational training covers. Deeper diving is a separate, technical discipline.
Pressure is absolute (air plus water) in seawater, rounded. Each bubble's area shows how much space a lungful of surface air takes at that depth. Depth limits vary by agency and course, so follow your own training.
Notice how much changes in the first 10 m. Air halves in volume between the surface and 10 m, then loses only another sixth or so by 20 m. That is why the shallows need the most care. The theory guide explains more.
Guides on this site
How certification courses are structured
The three parts of a first course, what you need before you start and what the card lets you do.
02The theory part of a course
Pressure, breathing gas, planning and the environment, and how to get the most from e-learning.
03Pool and confined water skills
The core skills you practice in shallow water before your first open water dive.
04Open water training dives
What happens on the dives that finish a course, from briefings to logging.
05Scuba gear basics
What each piece of equipment does and what beginners usually own or rent.
06Dive safety rules
The rules every new diver learns, and why you never dive beyond your training.
07Fitness to dive
Why schools ask for a medical form, and why the answer comes from a doctor.
08Choosing a dive school
Questions to ask, what to look at and the warning signs worth walking away from.
09After certification
Keeping skills fresh, building experience slowly and when to take a refresher.
10Scuba glossary
Plain definitions of the words you will meet in your course materials.
The rule we repeat on every page
Dive within your training and experience. A website can explain how diving works, but it can't check your skills, your gear or your fitness. A certified instructor and your training agency's own materials are the right sources for anything you will do underwater, and a doctor is the right person to ask about fitness to dive.
Common questions
Can I learn to dive online?
You can learn the theory online. Many courses now offer the knowledge part as e-learning that you finish before you meet your instructor. The in-water parts can only be learned in the water, with an instructor watching. No website or video replaces that.
Do I need to be a strong swimmer?
You need to be comfortable in the water. Courses usually include a swim and a float test before or during training. Ask the school what theirs involves, and practice in a pool beforehand if you are unsure.
Which training agency is best?
We don't rank or recommend agencies. Several international agencies train recreational divers to broadly similar standards, and their cards are widely accepted. The instructor and the school often matter more to your experience. See choosing a dive school.