To make it super simple- a surface marker buoy shows boaters or people watching on the surface, that a diver is down underwater. A simple tube inflated with air thirst is floated on the surface during a dive, to mark the divers position during dives.

The Mares Surface Marker Buoy is an item that every diver should carry with them, during night dives, ocean dives in restricted visibility such as sea fog, mist or disturbed sea conditions.

An Surface Marker Buoy or SMB is also absolutely essential for diving anywhere you may dive in an area that has surface boat traffic, ta surface marker buoy could potentially save your life, by alerting people on the surface, to exactly where you are in the water, should you get into a bad situation.