A Surface Marker Buoy, SMB or simply a blob is an inflatable buoy used by SCUBA divers, with a line, to mark the diver's position to their surface, safety boat whilst the diver is underwater.

Surface marker buoy on a line and reel

SMBs are inflated on the surface before diving to mark the diver's position in these circumstances:

• during a drift dive or night dive so the dive boat can follow the divers
• where there is boat traffic making diving more hazardous

Divers need to consider some configuration options and features when using SMBs:

• A closed SMB, with a valve which you blow through, is likely to be more reliable, by remaining inflated, than an open ended buoy or a delayed buoy which seals itself as it inflates
• To avoid losing the reel, the reel needs a lanyard to attach the diving reel to the diver: this can :
  • either clip to the buoyancy compensator or go around the wrist
  • or be long enough to float above the diver and stay out of the way. If the lanyard clips to the BC, take care to release if there are boats around (boats on the surface have been known to drag divers up by their SMB reels).
    The DIR diving philosophy generally considers unsafe any attachment to equipment or objects which end above the water surface, due to high risk associated with dragging the diver upwards in spite of their decompression obligation or maximum ascent speed limit.

A specialised form of SMB is the decompression buoy.