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. When used by a diver to indicate their position, any of these may be described as a personal marker buoy.
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.
