The SI or surface interval is the amount of time between 2 dives which is used to calculate the limits in consecutive dives.
A DSMB or Delayed Surface Marker Buoy works the same as a SMB but it is inflated and deployed from underwater. It is composed of a surface marker buoy secured by a line on a spool or by a WDS. DSMB allow a diver to be located and spotted from the surface while still being at desired depth. It helps in preventing and avoiding accidents with boats and is considered mandatory in many places.
A SMB or Surface Marker Buoy is a safety signaling device in the form of an inflatable tube of fabric brightly colored. SMBs are used at the surface to be easily visible and spotted from far away. Some can also be deployed from underwater and are called DSMB.