Float with a dive flag: A float with a dive flag is a necessity for spearfishing as it alerts boaters to the fact that there are divers in the area and they should not come within 50m of that dive flag. (This will prevent possibly fatal or crippling injuries caused by propellers, as well as ensuring your gun is not ripped from your hands if a boat unknowingly motors across your floatline.)
A further advantage of having a float and float line is that you will be able to let go of your gun when shooting large fish, knowing that you will be able to get to the surface for air while still retaining your gun and catch. Additionally, when shooting into caves, or around rocks and reef, your shaft may get stuck. Having a float and float line means that you are able to easily track down your gun, even in poor visibility or strong current.
For these reasons, it is easy to see why a float and clearly visible dive flag are requirements for entering spearfishing competitions.