Underwater foraging uk dyneema core bungee come in 3 lengths 1m,2m and 3m.
 
They can attach between your float line and your float or more effectively, between your float line and speargun.
The main thing to know about them is whichever size you order, they stretch 2 times longer when pulled before becoming fixed by the dyneema.  The dyneema has a breaking strain of 400lbs.
 
A massive benefit is when you’re in choppy conditions.  You dive down and the swell above starts yanking the float line and pulling on your gun.  This is especially frequent if you use a larger dive float like the patrol.  The bungee keeps this to a minimum as it’s the elastic that gets worked instead of your wrist.