Surface marker buoys (SMBs) are an essential accessory for marking your underwater position while diving. You may tow them behind you throughout the dive, or you may anchor them to the bottom or carry a marker buoy that can be deployed at the end of the dive, technically called a delayed surface marker buoy (DSMB). For this, you will need at least 15 meters/50 feet of synthetic line on a reel. (Never tie the line and/or float to your BCD or your body. You may get pulled up by the current or snagged.)
You will practice surface deployment in your last confined water session before going into open water. It is highly recommended you purchase your own SMB and reel or spool before going into the open water.