Question: When using propulsion while scuba diving, how is depth controlled?
About the article "Scuba diving", section "Diver propulsion".

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Personal mobility is enhanced by fins and optionally diver propulsion vehicles.
Fins have a large blade area and use the more powerful leg muscles, so are much more efficient for propulsion and manoeuvring thrust than arm and hand movements, but require skill to provide fine control.
Streamlining dive gear will reduce drag and improve mobility.
Balanced trim which allows the diver to align in any desired direction also improves streamlining by presenting the smallest section area to the direction of movement and allows propulsion thrust to be used more efficiently.
Occasionally a diver may be towed using a "sled", an unpowered device towed behind a surface vessel which conserves the diver's energy and allows more distance to be covered for a given air consumption and bottom time.
The depth is usually controlled by the diver by using diving planes or by tilting the whole sled.
Some sleds are faired to reduce drag on the diver.
Scuba diving is a form of underwater diving in which a diver uses a self contained underwater breathing apparatus (scuba) to breathe underwater.
Unlike other modes of diving, which rely either on breath-hold or on air pumped from the surface, scuba divers carry their own source of breathing gas, (usually compressed air), allowing them greater freedom of movement than with an air line or diver's umbilical and longer underwater endurance than breath-hold.
Scuba equipment may be open circuit, in which exhaled gas is exhausted to the surroundings, or closed or semi-closed circuit, in which the breathing gas is scrubbed to remove carbon dioxide, and the oxygen used is replenished from a supply of feed gas before being re-breathed.
A scuba diver usually moves around underwater by using fins attached to the feet, but external propulsion can be provided by a diver propulsion vehicle, or a sled pulled from the surface.  
Dive management skills.
Monitoring depth and time.
Whenever there is a possibility that the pressure exposure of a dive may incur a decompression obligation on the diver it is necessary for safety to monitor the depth and duration of the dive to ensure that either there is no decompression obligation, or that the appropriate decompression procedures are followed for a safe ascent.
This process may be automated by using a personal dive computer, in which case the diver is required to understand how to read the output and follow the decompression instructions displayed.
The display and operation of dive computers is not standardized, and the user is expected to learn the correct operation of the specific model of computer to be used before diving with it.
Accurate monitoring of depth and time is particularly important when diving using a schedule requiring decompression according to decompression tables. 
The equipment used for deep diving depends on both the depth and the type of diving.
Scuba is limited to equipment that can be carried by the diver, or is easily deployed by the dive team, while surface supplied diving equipment can be more extensive, and much of it stays above the water where it is operated by the support team.
Scuba divers carry larger volumes of breathing gas to compensate for the increased gas consumption and decompression stops.
Rebreathers manage gas much more efficiently than open circuit scuba, but are inherently more complex than open circuit scuba.
Use of helium-based breathing gases such as trimix reduces nitrogen narcosis and stays below the limits of oxygen toxicity.
A diving shot, a decompression trapeze or a decompression buoy can help divers return to their surface safety cover at the end of a dive.
Decompression can be accelerated by using specially blended breathing gas mixtures.
Surface supply of breathing gases reduces the risk of running out of gas.
In-water decompression can be minimized by using dry bells and decompression chambers.
Hot-water suits can prevent hypothermia due to the high heat loss when using helium based breathing gases.
Diving bells and submersibles expose the diver to the direct underwater environment for less time, and provide a relatively safe shelter that does not require decompression, with a dry environment where the diver can rest, take refreshment, and if necessary, receive first aid in an emergency.
Breathing gas s reduce the cost of using helium based breathing gases, by recovering and recycling exhaled surface supplied gas, analogous to rebreathers for scuba diving.
The most radical equipment adaptation for deep diving is to isolate the diver from the direct pressure of the environment, using armoured atmospheric diving suits that allow diving to depths beyond those currently possible at ambient pressure.
These rigid, articulated exoskeleton suits are sealed against water and withstand external pressure while providing life support to the diver for several hours at an internal pressure of approximately normal surface atmospheric pressure.
This avoids the problems of inert gas narcosis, decompression sickness, barotrauma, oxygen toxicity, high work of breathing, compression arthralgia high pressure nervous syndrome and hypothermia, but at the cost of reduced mobility and dexterity, logistical problems due to the bulk and mass of the suits, and high equipment costs.  
There are alternative methods that a person can use to survive and function while underwater, including:   free-diving - swimming underwater on a single breath of air.
snorkeling - a form of free-diving where the diver's mouth and nose can remain underwater when breathing, because the diver is able to breathe at the surface through a short tube known as a snorkel.
surface-supplied diving - originally used in professional diving for long or deep dives where an umbilical line connects the diver with the surface providing breathing gas, and sometimes warm water to heat the diving suit, and usually nowadays voice communications.
Some tourist resorts now offer a surface-supplied diving arrangement, trademarked as Snuba, as an introduction to diving for the inexperienced.
Using the same type of equipment as scuba diving, the diver breathes from compressed air cylinders, which float on a free floating raft at the surface, allowing the diver only 20–30 feet (6–9 m) of depth to travel.
Atmospheric diving suit - an armored suit which protects the diver from the surrounding water pressure.
Liquid breathing - so far, in the real world, liquid breathing for humans is only laboratory experiments, and (one lung at a time) medical treatment.
It has possibilities of being used for very deep diving.
It is memorably portrayed in the film "The Abyss".
Artificial gills - these are hypothetical.
They would have to process a huge volume of water to extract enough oxygen to supply an active diver, and processing this much water takes a great deal of energy (possible for cold-blooded fish, but harder for humans with higher metabolic rates).