All dives begin with setting up gear. The basic kit for a recreational scuba diver is the air cylinder (or tank), regulator, buoyancy control device (BCD), fins, mask, weight belt and wet suit (in all but the warmest tropical waters). Snorkels and dive computers are helpful, but not absolutely essential. This procedure should begin by trying on the wetsuit, fins and BCD if they have been rented, to ensure a good fit. Then the air cylinder is fitted to the BCD, and the regulator screwed on to the cylinder. The regulator's feed hose for the BCD is attached, and then all the cylinder is opened and all the equipment is checked out. Both the primary and emergency valves (the mouthpieces that are used for breathing) are checked, as is the pressure gauge to make sure the cylinder is full. With all of that done, the cylinder is closed so that an accident does not cause all the compressed air to leak out, and the various hoses clipped, tied, or folded up into the BCD for their protection. All of this should be done with care, especially on dives off of small boats. Making adjustments in a boat bouncing on choppy waters or in the water is awkward, and can be avoided just by spending a little time on proper set-up at the dive shop.
There should always be a short planning session before any dive. This is where the dive master (or the group, if there is no dive master) will outline how the dive will proceed. For example, a common plan for a pinnacle or group of rocks is to go straight to the bottom or low point of the dive, and then ascend in a spiral pattern around the undersea formation from there. This means everyone has a general idea of what the group is doing, and they can therefore spend more of their attention on things like sea life. If buddies have not been assigned yet, they should be now. Scuba divers always operate in pairs or trios for safety. This should also be when the air limit is set. Dives usually end, either for the whole group or for one set of buddies, when the first diver has drawn down their air to a certain point. This is commonly either 50 to 70 bar in a 220 bar cylinder.
After hitting the water and linking up with their buddies, scuba divers start deflating their BCDs and sinking. Buddies should keep an eye on each other, stay together and only descend at the rate of the slowest diver. The main snag for going down fast is equalization. As depth increase, so does water pressure, which places the squeeze on the air pockets in the head. Divers adjust for this by clamping the nose and blowing on it, chewing, or swallowing. All of these actions push more air into these spaces, equalizing the pressure. However, not everyone can do this at the same rate and some are just noticeably slower than others.
Once the divers are safely down, they can begin their dive according to plan. Divers should swim with their arms tucked in and their legs placed directly behind their body, to minimize drag, They should also avoid overexertion, since anything that causes hard breathing also uses up air more rapidly and cuts time off the dive. Small adjustments in buoyancy will be made throughout the dive using the BCD. It may also be necessary to clear the dive mask of fogging or water, which is a simple procedure taught in all basic dive certification programs.
When divers start reaching their pre-set remaining air limits, they do one of two things. If there are divers who still have plenty of air left (some have reached the limit of 50, but others are still at 90 for example), the group may re-organize to allow those divers with lots of air left to stay down and look around some more. Otherwise, everyone starts ascending to the surface. An important diving safety step is to keep breathing and never hold one's breath. The air in a diver's lungs is compressed, and holding it while making a sudden change in pressure can cause severe injuries as that compressed air expands under reduced water pressure. It is also normal for divers to make a safety decompression stop for several minutes at somewhere between 15 and 30 feet of depth. This allows the body to dispose of enough nitrogen in the blood to avoid any potential decompression sickness. After the safety stop, divers go to the surface, inflate their BCDs to stay afloat, and wait for pick-up.