Line and Reels
Probably the most common application for a reel used by recreational divers is for the deployment of a Delayed Surface Marker Buoy (DSMB). For this task, it must be relatively easy to release the line to freewheel and there must be little chance of it snagging. Caution: Never have the reel physically attached to you while deployment takes place.
A Float Line allows you to shoot at any depth and let go of your gear for a free ascent to the surface. With your speargun and shaft (or just the shaft with a break away rig) attached to a Float Line and then to a buoy on the surface, you can fight the fish from the surface.