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 submersible marker buoys that can be deployed at the end of the dive. For this, you will need at least 15 meters/50 feet of polypropylene rope 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 up.)
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 before going into the open water.