Question: How is the depth controlled when scuba diving using propulsion?
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. 
Scuba diving is very good around the island. 
Cave diving is underwater diving in caves which are at least partially filled with water.
The equipment used varies depending on the circumstances, and ranges from breath hold to surface supplied, but almost all cave diving is done using scuba equipment, often in specialised configurations.
Cave diving is generally considered to be a type of technical diving due to the lack of a free surface during large parts of the dive, and often involves decompression.
In the United Kingdom it is an extension of the more common sport of caving, and in the United States an extension of the more common sport of scuba diving.
Compared to caving and scuba diving, there are relatively few practitioners of cave diving.
This is due in part to the specialized equipment (such as rebreathers, diver propulsion vehicles and dry suits) and skill sets required, and in part because of the high potential risks, including decompression sickness and drowning.
Despite these risks, water-filled caves attract scuba divers, cavers, and speleologists due to their often unexplored nature, and present divers with a technical diving challenge.
Underwater caves have a wide range of physical features, and can contain fauna not found elsewhere.