There may be slight differences in language between various parts of the world
SMB - Surface Marker Buoy. Inflated prior to the dive, attached to a reel. As a diver descends he pays out line from the reel. On ascent, he/she reels in the slack line. Provides permanent indication of the divers position throughout the dive.
DSMB - Delayed Surface Marker Buoy. Like an SMB,
except, inflated underwater by the diver prior to ascent. Often used on a wreck dive, where the divers location is known under water, i.e. they are on the wreck. Deployed as they leave the wreck and drift clear. Especially useful if the diver will drift some distance prior to surfacing (high currents or decompression stops).
Flag - Generally a folding flag. Used for diver - boat separation. A divers head is a small object to see from a boat. A DSMB helps considerably, a flag is even better.
P.S. Experiments have been carried out to see what is easier to see when searching for a diver. The RNLI and Coastguard test concluded.
- Orange is easier seen when looking form a boat, i.e. when low on the water. Generally the orange stands out better against the sky. In bright sunlight it was often seen as a dark line against the pale / bright sky.
- Yellow was easier from the helicopter. i.e. when looking down form height. Generally the Yellow stands out better against the sea. A pale item, on a dark sea.