A decompression buoy, deco buoy or delayed SMB is an item of diving equipment used by SCUBA divers to mark their position whilst underwater doing decompression stops. They are similar to Surface Marker Buoys but deco buoys are launched whilst the diver is submerged. Alternative solutions to marking one's position while doing decompression stops are diving shots and decompression trapezes.

Deco buoys are inflated underwater before or during the ascent phase of the dive. A reel and line connect the buoy on the surface to the diver beneath the surface. The buoy marks the diver's position underwater so the boat safety cover can locate the diver.

Packed deco buoy

There are at least four methods of keeping the air in the inflated deco buoy. The buoy can be be:

• open ended (preferably with small independent weight to keep the opening submerged)
• open ended self sealing buoys (the air in the buoy expands as the buoy ascends closing a neck at the bottom of the buoy)
• sealed, with an inflation valve and a pressure relief valve
• sealed, with a built in air supply and a pressure relief valve

Divers 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, 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. Some types of buoy provide an attachment for a strobe, cyalume stick or writing slate, which can convey signals to the surface support.

Deco buoy ready to be inflated

Several common problems are encountered when deploying deco buoys :

1. The diving reel jambs after the buoy is inflated (dragging the diver up). To avoid this:
   • use a simpler system or a reel which cannot jamb (e.g. a weighted spool of line)
   • detach the lanyard connecting the diver to the reel before inflating the buoy (and ensure no equipment is trapped in the buoy or reel)
   • attach two reels to each other in series. If one fails the other is unlocked to reel out its line.
2. Part of the diver's equipment gets trapped in the deco buoy (dragging the diver up). To avoid this tie the lanyard of the reel to something solid on the sea bed before inflating the buoy (so you have time to sort the problem out)
3. The diver removes the primary demand valve from his or her mouth to inflate the buoy, and is therefore at a disadvantage in dealing with any other problems that might arise as the deco buoy goes off. There are a number of ways to avoid this:
   • Use a deco buoy with its own air supply
   • Use a secondary demand valve, such as an octopus, to inflate the buoy
   • Use a sealed buoy with an inflation valve, which is filled by blowing directly into the valve inlet or by attaching a medium-pressure inflation hose from the buoyancy compensator or dry suit. The valve does not retain the hose connector, like the BCD or suit inflator valve, and the hose can be easily pulled off the valve when the buoy is sufficiently filled.
   • Hold an open ended buoy above the primary demand valve and direct several exhalations up into the open end of the buoy. This technique is also useful in cold conditions to prevent freeflows caused by pressing purge buttons.