Surface marker buoy

A surface marker buoy, SMB, dive float or simply a blob is a buoy used by scuba divers, at the end of a line from the diver, intended to indicate the diver's position to people at the surface while the diver is underwater. Two kinds are used; one (SMB) is towed for the whole dive, and indicates the position of the dive group throughout the dive, and the other, a delayed surface marker buoy, DSMB or decompression buoy, is deployed towards the end of the dive as a signal to the surface that the divers have started to ascend, and where they are going to surface. Both types can also function as a depth reference for controlling speed of ascent and accurately maintaining depth at decompression stops. Surface marker buoys are also used by freedivers in open water, to indicate the approximate position of the diver when submerged. They may also be used to support a catch bag or fish stringer by underwater hunters and collectors. A DSMB is considered by recreational scuba divers and service providers to be a highly important item of safety equipment, yet its use is not part of the entry level recreational diver training for all training agencies, and there are significant hazards associated with incompetent use.
A "safety sausage" or "signal tube" is a low volume tubular buoy inflated at or near the surface to increase visibility of the diver in the water. A DSMB can be put to this service when necessary.
Another function for a buoy deployed by a diver is to mark the position of an underwater point of interest. In this use the buoy is attached to the target until the position has been recorded, or until the marking function is no longer required. A wider range of equipment is used for this function, including the same equipment that would normally be used for marking the position of the diver. A problem associated with this use is how to hold the buoy in position at the point of interest but still be able to retrieve it from the surface.
Standard buoy.
Primary function.
Surface marker buoys are floated on the surface during a dive to mark the diver's position during drift dives, night dives, mist or disturbed sea conditions such as Beaufort force 2 or greater. The buoy lets the dive boat follow the divers and highlights their position to other boat traffic which makes it easier to stay clear. In some waters divers may be required to use a float to mark their presence. The US Coast Guard has conducted a public education campaign to get divers to add identification information to their dive-floats, to help them identify and find lost divers, and so divers who lost their floats can advise the Coast Guard that, if found, their float should be regarded as a false alarm.
Other uses.
Occasionally an SMB is used in conditions of poor visibility where there is a risk of inadvertently penetrating an overhead environment, to ensure that there is a guideline leading out of the overhead to the surface. 
In an emergency where buoyancy control is lost due to a BCD leak, the SMB can be used to compensate for buoyancy loss to the extent of the buoyancy of the surfaced buoy. This may require considerably more effort to wind in the line, but finning upwards will help. The buoyancy of any SMB can be used to help maintain a constant depth at decompression stops, and the process of winding in the line can help maintain a steady ascent rate. 
Competitors in the underwater sport underwater orienteering are required to tow a SMB with a buoyancy of at least during competition swims. This is both for safety, and to allow the judges to monitor the route taken by the diver and to score the points for time and accuracy. Such SMBs are designed for low drag, which is a useful feature in any SMB that will be towed by the diver.
A GPS tracker can be mounted on the SMB to record a dive track. This can be downloaded and used to establish positions of underwater landmarks with reasonable accuracy depending on surface conditions and current. Position of a point of interest can be established by the timestamp on a photograph taken of the feature if the camera clock is synchronised with the GPS unit. The position will be most accurate when the buoy line is tensioned to float as close as possible to directly above the diver.
Spearfishers also use surface marker buoys to mark the position of the speargun in case it is necessary to let go after spearing a fish or for any other reason. These are towed on a line attached to the speargun handle. Similar buoys with catch bags are used by freedivers for other underwater hunting and gathering activities. They serve as a place to gather and transport the catch, and may be equipped with a means of hooking to the bottom to stop them drifting away while the diver is busy.
A surface marker buoy is considered an essential tool in the initial project phases for establishing a coral nursery, when exploratory dives are being conducted to find suitable sites for nurseries, donor sites and transplantation sites. The SMB is used to mark these areas until a GPS position can be recorded.
Construction.
Buoys for this use are usually either inflated and sealed by a valve or cap, or made from buoyant material, so they cannot deflate or flood during the dive, rendering them ineffective. High-visibility colours such as red, orange and yellow are popular. Sometimes the float includes a small diving flag. If the buoy is to be towed by the diver at any speed, a low drag float and small diameter line can reduce the drag significantly. The torpedo buoys used by lifesavers are sometimes used as surface marker buoys as they are visible, tough, available, and reasonably low drag. 
To avoid losing the reel, a lanyard may be used to attach the diving reel to the diver. This lanyard can clip to the buoyancy compensator or go around the wrist. If the lanyard clips to the buoyancy compensator, the user should take care to release it if there is surface boating activity, as boats may drag divers up by their SMB reels. The DIR diving philosophy considers unsafe any attachment of the diver to equipment or objects which end above the water surface in waters where boats may operate, due to high risk associated with snagging the object on a boat and dragging the diver upwards in spite of their decompression obligation or maximum ascent speed limit.
Decompression buoy.
A delayed surface marker buoy (DSMB), decompression buoy, or deco buoy is an inflatable buoy which can be deployed while the diver is submerged and generally only towards the end of the dive. The buoy marks the diver's position underwater so the dive boat crew can locate the diver even though the diver may have drifted some distance from the dive site while doing decompression stops. A reel and line connect the buoy on the surface to the diver beneath the surface. A DSMB can help the diver maintain accurate depth during a decompression stop. Alternative means of marking one's position while doing decompression stops are shot-lines, uplines and decompression trapezes.
A closed DSMB, inflated through a valve, is likely to be more reliable, by remaining inflated, than an open ended buoy which seals by holding the opening under water. A decompression buoy is not intended to be used to lift heavy weights: for this purpose divers use a lifting bag.
Design.
There are at least four methods of keeping the air in the inflated deco buoy. The buoy can be:
Divers following the recommendations of some training organisations carry two differently coloured deco buoys underwater so that they can signal to their surface support for help and still remain underwater decompressing. For example, in some circles in Europe, a red buoy indicates normal decompression and a yellow buoy indicates a problem, such as shortage of gas, that the surface support should investigate and resolve. Although in other circles, two buoys (any colour) up one line means the same, currently the protocol is not universally accepted even within Europe. While the usual colours are red, yellow and orange, bright pink, lime green, bicoloured red and yellow, and black buoys are also available and may show up well in particular circumstances. It is quite common to have one or more stripes of reflective tape, which reflect light back towards the source. This works well if searchers have a good light source. 
Some types of buoy provide an attachment for a strobe light, cyalume stick or writing slate, which can convey signals to the surface support. Reflective tape may be used to make the buoy more visible at night.
Length is usually from . Visibility at a distance in waves is largely determined by height.
The size of a decompression buoy is to some extent at the diver's discretion. A tall buoy is more visible in rough water, and a larger diameter is visible at a greater distance. A large volume holds more gas and is more buoyant, which is helpful for some purposes, but is a greater hazard during deployment in the event of a reel jam.
Deployment practice.
Reliably safe deployment in difficult conditions depends on sufficient practice and familiarity with the equipment and the specific technique to be used for inflation.
Deployment problems and hazards.
Several problems may be encountered when deploying decompression buoys.
Safety sausage.
A safety sausage or signal tube is an inflatable buoy used when the diver is at the surface to indicate the diver's position to the dive boat, reducing the risk of losing contact when air, light or sea conditions decrease the visibility of the divers from the boat. The sausage is a plastic tube that is normally inflated by putting one end under water and purging the second stage underneath to inflate it. Inflated tubes are normally about tall. Uninflated sausages roll up and fit in a buoyancy compensator pocket. Commercial boat dive operations, especially at offshore reefs or areas known for strong currents or mercurial weather, may require divers to carry safety sausages. A safety sausage is not a substitute for a surface marker buoy or diver down flag, though some divers use the term safety sausage to refer to a DSMB as well as a signal tube.
Point of interest markers.
Small deployable marker buoys are available that are provided with a length of line wrapped around the hard foam buoy, and with a small weight which almost balances the buoyancy. These can be deployed by a diver to mark a point of interest and can be recovered from the surface after the dive. They are deployed by releasing the weight from the buoy and allowing the line to unroll as the buoy floats to the surface. If the line is long enough the buoy will float at the surface with slack in the line and the weight will anchor it in place. If the line is too short the buoy will support the weight above the bottom and drift away.